his goals have shifted and changed for the worse since the heresy but I genuinely felt bad for fabius in the horus heresy, the dude has space cancer because of a flaw in the third legion gene seed, the flaw was fixed in all future ec thanks to a new stock from fulgrim but everyone who was already infected was fucked, the only reason fabius is still alive was because he fudged the lists so he wouldn't be on them, so he starts learning everything he can about the geneseed and starts making some real headway, he's learned how to harden the bones or toughen the skin, he starts figuring out how the organs work and how to improve them, then fulgrim finds out, and he approves, he endorses biles work, of course by this point everyone in the leadership is chaos tainted and biles work goes strange so instead of improving the marines for the better he begins doing things like killing their empathy or making it so they can only feel pleasure, by the end of the horus heresy he's altered most of the third legion and cloned primarchs, but he's still dying.

tl;dr Fabius turned traitor because he didn't want to die thanks to the emperors sloppy genework.