A particularly harsh English proverb along these lines appears in the following form in Wolfgang Mieder, A Dictionary of American Proverbs (1992):
Save a thief from the gallows and he'll be the first to cut your throat.
Versions of this proverb go very far back in English writing. Bartlett Whiting, Proverbs, Sentences, and Proverbial Phrases from English Writings Mainly Before 1500 (1968), lists instances dating to circa 1300, in an entry worded as follows:
Deliver a thief from the gallow(s) and he hates you after
The sense of the expression is that a person of bad character is inherently ungrateful to would-be benefactors and exceedingly unlikely to reform if given another chance. James Kelly, A Complete Collection of Scottish Proverbs: Explained and Made Intelligible to the English (1721) adds a real-life instance to his rendition of the proverb:
Buy a Thief from the Gallows, and he'll help to hang your self.
I knew a very worthy Clergyman in Scotland, who, by his Interest and Importunity, saved a Villain from the Gallows: And twelve Years after, he was the first that rabbled him, and the sorest upon him.
(A subsequent edition of Kelly's book (1818) reports that an English equivalent of the saying is "Put a snake in your bosom, and it will sting when it is warm.")
That such a person should repay kindness with unkindness is hardly surprising. And that such a person is—according to folk wisdom—unlikely to reform is evident in a kindred proverb that appears in Thomas Fuller, Gnomologia: Adagies and Proverbs; Wise Sentences and Witty Sayings (1732):
The Thief is sorry he is to be hanged, but not that he is a Thief.
Oswald Dykes, English Proverbs, with Moral Reflexions (1713) devotes four pages of exposition to the proverb, including a more general discussion of ingratitude:
Save a Thief from Hanging, and he'll cut your Throat.
It [the proverb] is as severe a Lecture also against doing an unthankful Person a Kindness, as against saving a Thief from the Gallows. There is as much Imprudence in the one, as Danger in the other ; for nothing can engage an Ingrate against abusing his Benefactor, or a Thief unhang'd against cutting his Friend's Throat.
'Tis impossible to oblige an ungrateful Wretch ; so that Favours are but thrown away upon him, and lost for ever. If you respect him, he'll slight or revile you ; if you relieve him, he'll bring you into Trouble ; and if you save his Life, in short, he'll endanger yours for't. These are the Returns he usually makes his best Friends.