The OED’s view of ‘myrmidon’
The Compact Edition Oxford English Dictionary (1971) lists the following meanings of myrmidon:
1. (With a capital M.) One of a warlike race of men inhabiting ancient Thessaly, whom, according to the Homeric story, Achilles led to the siege of Troy (Iliad II, 684). [Examples omitted.] b. Used of Achilles himself. [Example omitted.] 2. transf. A soldier of (one’s) body-guard; a faithful follower or servant. ? Obs. [Examples omitted.] 3. An unscrupulously faithful follower or hireling ; a hired ruffian ; a base attendant. [Examples omitted.] b. Chiefly myrmidon of the law, of justice : applied contemptuously to a policeman, bailiff, or other inferior administrative officer of the law. [Examples omitted.]
As tchrist points out in a comment beneath the OP’s question, the OED gives as its earliest instance of a citation illustrating definition 3, a 1649 quotation from Milton (Eikonoklastes iv. 30). Here is a fuller version of that quotation:
The parliament moreover had intelligence, and the people could not but discern, that there was a bitter and malignant party grown up now to such a boldness as to give out insolent and threatening speeches against the parliament itself. ...
The parliament also, both by what was discovered to them, and what they saw in a malignant party, (some of which had already drawn blood in a fray or two at the court-gate, and even at their own gate in Westminster-hall,) conceiving themselves to be still in some danger where they sate, sent a most reasonable and just petition to the king [Charles I], that a guard might be allowed them out of the city, whereof the king’s own chamberlain, the Earl of Essex, might have command ; it being the right of inferior courts to make choice of their own guard. This the king refused to do ; and why he refused the very next day made manifest : for on that day it was that he sallied out from Whitehall, with those trusty myrmidons, to block up or give assault to the house of commons.
The complication here is that the original word Myrmidon bears carries elements of aggressiveness and unquestioning loyalty that are quite compatible with much of the OED’s definition 3. Here is the entry for Myrmidons in William Benet, The Reader’s Encyclopedia, second edition (1965):
Myrmidons. In classic mythology, a people of Thessaly. They followed Achilles to the siege of Troy, and were distinguished for their savage brutality, rude behavior, and thirst for rapine. They were originally ants, turned into human beings by Zeus to populate the island of Oenone.
A closer look at the OED’s examples of definition 3
What definition 3 adds to Benet’s description (and to the OED’s own definition 2) of myrmidon are the hireling status and the elements of unscrupulousness and baseness. But in the quotation from Milton, though Milton clearly considers the party of royalist partisans at Westminster “bitter and malignant” (he uses the latter word twice in describing them), he does not, as far as I can tell, view them as being hirelings nor even as being especially unscrupulous or base. Their primary faults from Milton’s point of view are that they are as bellicose and utterly devoted to their leader (Charles) as were the Myrmidons in the Iliad in their service to Achilles, and that they support the wrong side.
The next several examples that the OED gives as instances of definition 3 suffer from the same problem: It is quite possible to understand myrmidon as being used (ironically or not) in the sense of the OED ‘s definition 2—“A soldier of (one’s) body-guard; a faithful follower or servant”—without any need to introduce the unscrupulousness, baseness, or hireling status of definition 3. These examples are (with a bit more surrounding text added for context), as follows. First, from Samuel Pepys’s Diary (July 12, 1666):
He [Sir William Coventry] spoke contemptibly of [Captain (and subsequently Rear-Admiral)] Holmes and his mermidons, that come to take down the ships from hence, and have carried them without any necessaries, or anything almost, that they will certainly be longer getting ready than if they had staid here.
Here, Holmes’s mermidons are simply subordinate officers and sailors in the British navy, and Coventry’s criticism is chiefly of Captain Holmes for moving the ships before they were properly outfitted and thus (in the bigger picture) prolonging the amount of time before the ships are ready to sail. I’m fairly sure that contemptibly is used here to signify “contemptuously.”
Next, from Henry Fielding, Tom Jones (1749):
But a more lucky circumstance happened for poor Sophia : another noise now broke forth, which almost drowned her cries ; for now the whole house rang with, ‘Where is she? D——n me, I’ll unkennel her this instant. Show me her chamber, I say. Where is my daughter? I know she’s in the house, and I’ll see her if she’s above ground. Show me where she is.’—At which last words the door flew open, and in came ‘squire Western, with his Parson, and a set of myrmidons at his heels.
In this scene it is unclear whether the set of myrmidons are in the employ of Western or of the lady whose home he has invaded—but either way, Fielding gives us no reason to suppose that any of them is anything but what OED definition 2 calls “a faithful follower or servant.”
And then from Thomas Love Peacock, Headlong Hall (1816):
Squire Headlong, in the mean while, was quadripartite in his locality ; that is to say, he was superintending the operations in four scenes of action—namely, the cellar, the library, the picture-gallery, and the dining-room,—preparing for the reception of his philosophical and dilettanti visitors. His myrmidon on this occasion was a little rednosed butler, whom nature seemed to have cast in the genuine mould of an antique Silenus, and who waddled about the house after his master, wiping his forehead and panting for breath, while the latter bounced from room to room like a cracker, and was indefatigable in his requisitions for the proximity of his vinous Achates, whose advice and co-operation he deemed no less necessary in the library than in the cellar.
With regard to Achates, Wikipedia reports that “In the Aeneid, Achates (‘good, faithful Achates’, fidus Achates as he was called) was a close friend of Aeneas; his name became a by-word for an intimate companion.” If we take seriously the echo of Achilles’ warlike retainers, there may be a note of jest in equating the little butler with the original Myrmidons; but I detect nothing of baseness or unscrupulousness in the man, and obviously Squire Headlong values his counsel.
Finally the OED cites Frederic Farrar, The Life of Christ, volume 2 (1874) in a situation where the “unscrupulously faithful follower or hireling” sense—if not the “hired ruffian” sense—of myrmidon seems arguably applicable:
For the second time Jesus is derided—derided this time as Priest and Prophet. Herod and his corrupt hybrid myrmidons “set Him at nought” — treated Him with the insolence of a studied contempt. Mocking His innocence and His misery in a festal and shining robe, the empty and wicked prince sent Him back to the Procurator, to whom he now became half-reconciled after a long-standing enmity.
It appears that the myrmidons in this case are those attendants of Herod whom Farrar elsewhere identifies as “the Chief Priests and Scribes.” Farrar’s prose is so overheated that you wouldn’t have to interpret the word myrmidon as meaning anything more than “faithful follower” to reach a sense of “corrupt hybrid myrmidons” that comes out fairly close to the meaning “unscrupulously faithful follower” or “base attendant” that the OED definition 3 attributes to myrmidon alone.
Myrmidons of law enforcement
Meanwhile, the OED’s definition 3(b) of myrmidon—“a policeman, bailiff, or other inferior administrative officer of the law”—had become well established in British English after originating, apparently, as slang coined by some impressively literate member of the canting crew (the street beggars, thieves, and vagabonds) of London. A New Canting Dictionary (1725) has this definition:
MYRMIDONS, the Constable’s Attendants, or those whom he commands (in the King’s Name) to aid and assist him : Also the Watchmen.
The OED attributes a shortened version of the same definition to a 1700 edition of B. E., A New Dictionary of Terms, Ancient and Modern, of the Canting Crew, which appears as the first example of the OED’s definition 3(b) in action. The same term appears many years later later in Francis Grose, A Classical Dictionary of the Vulgar Tongue, third edition (1796):
MYRMIDONS. The constable’s assistants, watchmen, &c.
One can easily imagine that denizens of the London underworld might use myrmidon derisively when applying it to the representatives of law and order with whom they had frequent contact; but definition 3(b)’s framing description “applied contemptuously” doesn’t seem any more apt in connection with myrmidon than with, say, beck (beadle), bus-napper (constable), bus-napper’s kenchin (watchman), hamlet (high constable), harman (constable), harman beck (beadle), Mr. Thingstable (constable), or night magistrate (constable)—all slang terms that appear in Grose’s 1785 and or 1796 dictionary—as well as such more recent terms as cop, copper, flatfoot, fuzz, and pig. The “applied contemptuously” comes with the territory. I suspect.
Early Google Books matches for ‘myrmidon’ in the OED definition 3 sense
In a Google Books search, the strongest candidate for an early instance of myrmidon used in the sense of OED’s definition 3 that I could find was not from any of the examples that dictionary offered, but from Susanna Centlivre, The Busy Body (1709):
Sir Jealous. What, is he in, then?
Marplot. Yes, sir, he is then ; and I say if he does not come out I have half a dozen myrmidons hard by shall beat your house all about your ears.
Sir Jealous. Ah! A combination to undo me—I’ll myrmidon you, ye dog, you—Thieves! Thieves! [Beats Marplot all the while he cries thieves.]
It is difficult to say whether the (imaginary) myrmidons that Marplot alludes to are supposed to be understood as hired ruffians or as agents of the constabulary, but Sir Jealous seems to take them for the former, and the situation in this comedy invites that interpretation as well.
Another interesting instance appears in The Scots Magazine (May 1778) in connection with an incident involving the American officer in charge of the captive British army under General Burgoyne, which had surrendered after the Battle of Saratoga:
New York, April 13 . It is reported, that the prosecution which Gen. Burgoyne lately carried on against the rebel colonel Henley, originated from the following cause. Col. Henley commanded the guard placed upon the said General, and the army under his command, whom he [Henley] and his myrmidons constantly abused with bitter invectives and scurrilous epithets. A British serjeant one day modestly remonstrated against such unmanly proceedings, was grossly insulted by Henley and his gang. The serjeant growing warm declared that none but miscreants would treat unarmed soldiers in so unworthy a manner, upon which Henley heroically drew his sword and run him through the shoulder.
Here the author of the report might be using myrmidons in the simple sense of “military subordinates,” but the fact that he also calls Henley’s subordinates a “gang” suggests that a much more pejorative sense of myrmidon is intended.
For what it’s worth, Merriam-Webster’s Eleventh Collegiate Dictionary (2003) doesn’t acknowledge the “hired ruffian” sense of myrmidon at all, though it does report the distinct possibility of there being an unscrupulous element in the follower’s conduct:
myrmidon n (15c) 1 cap : a member of a legendary Thessalian people who accompanied their king Achilles in the Trojan War 2 : a loyal follower; esp. a subordinate who executes orders unquestioningly or unscrupulously
The arc from belligerent supporter to loyal retainer to deputized enforcer to hired ruffian isn’t hard to follow, though each transitional step along the way is evidently more a matter of happenstance than of necessity, and though I am not at all persuaded—at least not on the strength of the OED’s first five citations on behalf of its definition 3—that the OED definition 3 sense of myrmidon is necessary to explain more than a few recorded instances of the word before, say, 1850.
The instances that I’ve looked at—both from the OED’s list of examples and from the results of a Google Books search—suggest two things: (1) that the notion of a follower who is “unscrupulously faithful” or a “hired ruffian” did not firmly attach itself to myrmidon until much later than the OED’s suggested first occurrence date of 1649; and (2) that the OED’s definition 3(b) may well have emerged in popular usage before its definition 3, in which case myrmidons of the constabulary may have been the inspiration for myrmidons of criminal enterprises.