>>281369030
Shinji's basically a regular bum who got mad with power: that's his premise. You also have his cursed past, with being sent off for education out of Fuyuki, witnessing both his parents die and be used as worm meat, and realizing that Sakura took away any chances he had of being the Matou mage. That enough is alone for him (with Zouken's push) to become a terrible person over the course of about 2-5 years and get the personality of your average school shooter, in a twisted sense.
Kirei, however, was doomed from the start to become evil. He didn't want to repress his inner urge to dismiss morality, and therefore lost his way. If anything, him being born in a priest family meant he could at least do his duties with dignity and give his external shell a reason to exist.
Zouken, at the time of FSN's story, had been insane for 4 centuries. Any noble goals he had in the past were buried deep under the growing insanity caused by him reaping the consequences of trying to be immortal as a means of attaining his greater goal.
Shinji has no internal sense of morals, as that was taken away from him when he felt injustice from having all that effort he spent learning magecraft go nowhere. He also has no greater aspiration, as his inferiority complex (that he covers up with his sense of superiority) tells him he's incapable of fulfilling one. He's a complete tool, and the only person who could feasibly fix him was Shirou, whom (as Nasu claims) was the only person he could feel natural talking with.
If it's any consolation, it's implied that Wakame made Gil kill Zouken during UBW's events.
>>281369137
This. Having to settle with a JP->CN->EN translation is absurd. Although I'm pretty sure Zero did get a TL at some point? Maybe that was the manga though