All your uses are fine as they go, but the last set is clearest because of how uncommon a word magistricide is. On the other hand, it is solidly built of well-established parts, so it would be immediately recognizable to any educated reader.
You would not be the first to use magistricide, for this is a rather old word, not a newly minted one as some have suggested. From the OED:
Etymology: f. as if L. *magistricīda (after parricīda, etc.: see -cide 1), f. magister master.
A murderer of one’s master or teacher.
- 1670 Lassels Voy. Italy II. 172 ― Nero the Magistricide, who put this rare man his master to death.
Notice the OED calls it a nonce word: a word crafted for one particular use at one particular time and used just that once. Apparently the 1670 citation is not wholly unique, which makes it more of a rare word than a nonce word. Still, even if it is rare, it is perfectly clear in its meaning.
Even if your word were actually freshly minted, it is a only natural to form new words based on old models. That’s how language works. Indeed, the ‑cide component has been used time and time again to form new nouns, and sometimes adjectives, that work like suicide and homicide — which, by the way, are the only two that have come into common use as verbs not just as nouns.
Here are some 126 ‑icide words mentioned in the OED:
acaricide, algicide, aliicide, apricide, arboricide, avicide, bacillicide,
bactericide, bioinsecticide, biopesticide, bovicide, Brahminicide,
canicide, cervicide, ceticide, Christicide, coquicide, culicicide,
cyanicide, deericide, deicide, dominicide, elephanticide, episcopicide,
famicide, felicide, femicide, feminicide, feticide, fideicide, filicide,
floricide, foeticide, fratricide, fungicide, gallicide, gallinicide,
gelicide, germicide, giganticide, gregicide, herbicide, hereticide,
hericide, homicide, hospiticide, hosticide, hymnicide, infanticide,
insecticide, lapicide, larvicide, legicide, leporicide, liberticide,
libricide, lignicide, macrofilaricide, macropicide, magistricide,
matricide, menticide, microbicide, mildewicide, miticide, modernicide,
molluscicide, monstricide, muricide, mycoherbicide, naticide, nematicide,
neonaticide, omnicide, ovicide, oxyuricide, paracide, parasiticide,
parenticide, parricide, patricide, perdricide, pesticide, philosophicide,
piggicide, piscicide, politicide, populicide, prolicide, raticide,
ratticide, regicide, regnicide, rodenticide, schizonticide, self-homicide,
senicide, serpenticide, siblicide, silicide, silvicide, sororicide,
spermaticide, spermicide, stillicide, stiricide, suicide, taeni(i)cide,
talpicide, tauricide, temporicide, tickicide, tsaricide, tyrannicide,
urbicide, ursicide, utricide, uxoricide, vaccicide, vaticide, verbicide,
vermicide, verminicide, viricide, vulpicide, and weedicide.
There are also a few ‑ocide words:
biocide, ecocide, ethnocide, genocide, Negrocide, phytocide, pseudocide, spermatocide, trypanocide, and tuberculocide.
What all that shows is that people are rather fond of coining new ‑cide words. I find many of those listed significantly less recognizable than your magistricide, and I feel that some of them work better than others. For example, why someone would elect weedicide over herbicide, I have no earthly idea. Piggicide is another ungainly and unfortunate term, but I suppose they must have feared that suidicide might be confused for suicide.
Nonetheless, I would suggest using magistricide in a context where the connection between master and magister were made eminently clear, such as in the Lassels citation or indeed in your own final proffered set of sentences:
- The master noticed Bob’s failed magistricide attempt.
- The master noticed Bob’s failed attempt at magistricide.
- The master noticed Bob’s failed attempt of magistricide.
As with the Lassels, by using both the current English word and the learnèd Latinate one together, you gently remind your reader of the connection between them.