I agree with @jneapan that this should probably be read おのれ, but if you’re interested in a bit of rambling (I feel like I’m actively turning this thread into a “short questions long answers” thread but well)… basically, a good rule of thumb is that for classical pronouns that have both a short and a long form (わ・われ、あ・あれ、な・なれ、おの・おのれ), use the short form before が or in compounds, and the long form otherwise.
Rambling
Well, first, you can find many different readings for 己 in the dictionary but they’re all variants of おの（れ）, which is the original version, the (classical) pronoun “oneself” (modern 自分). So the question boils down to whether it should be read おのれ (long form) or おの (short form).
As a general rule, except in very ancient texts, the short form of pronouns was used only before the genitive and in compounds. However, another thing is that in the old language, personal pronouns, including おの normally marked the genitive with が and not の. However, at some point, the short forms became kind of lexicalised as possessive pronouns by fusing with the が, and the long forms became the main stand-alone words. Therefore I don’t think it’s shocking to find おのれの instead of おのが, especially in modern prose, although the use of おのれ in a modern text feels archaic in itself, maybe おのがため would feel even more archaic.
BTW, it’s related to this answer I wrote in another post about あ（れ） vs わ（れ）, if you want to read more about archaic pronouns…