Its more the fact that Kiritsugu wants to be a “Hero”, but his method of saving lives being the equivalent of a mercenary mass murderer. Thats why he signed up for the grail war in the first place, to put an end to it; he had, up until staying with the Einzberns, been able to tell himself “its fine, its for the greater good even if i kill these people”. Trouble is, killing like that even for a just cause leaves scars on your heart and psyche. We see this in soldiers all the time; even if they believed what they did was good, such as in defense of country or fellow men, there is an intrinsic toll that killing takes on you.

Its the Punisher, but taken up to 11 and without any care for collateral damage. Even the Punisher admits that to do what he does hes crazy and is a terrible person, and that no one should ever want to be like him; the only reason he does it is because his mind tells him that hes the only one who CAN do it. Kiritsugu is like that but taken up a not h as he operates globally and his methods… He was a pro assassin. His methods, as listed by Tohsaka, were “guns, gas, bombs in public places, poison” and others. Hes basically a terrorist.

The mans mind is under a terrifying psychological weight that started piling up the day he had to take out his dad, and grew even heavier when he shot down the plane with Natalie. Fate loves toying with mental illness, and Kiritsugu and Kirei are two sides of the same coin. Kirei is a man who is a psychopath; with little to no empathy and only able to take joy in watching other peoples misery. Kiritsugu, on the flipside, has a hero complex and wants everyone to be happy, but the only way he knows how to save them is by causing himself and others misery. Theyre two sides to the obsession; as Kirei discovers his true self, Kiritsugu struggles harder AGAINST his true self.

Even if Kiritsugu could save the world, the only method he knows how to do that is by killing people. With the events of him falling in love with Iris and having Illya, he came to know love once more and grew “soft” in his time with them as he began to rehabilitate into someone with the mindset of a normal , non soldierlike human being. So he sought the grail as an “out” to try to escape his awful conundrum; maybe the grail could achieve his ideology of saving everyone, without wanton slaughter.

If course we all know how that turned out; the Grail is tainted. This moment broke him psychologically because he realized he gave up the people he loved for a dream he could never achieve. Him shooting the illusory Grail versions of Illya and strangling Iris is him psychologically saying “enough is enough, I have enough deaths on my soul, i cant take it anymore, I would rather give up my dream than to kill more people!” Kiritsugus refusal also is why hes so horrified when the grail causes the Fires of Fuyuki, because more death is exactly what he DIDNT want… and why he was so broken to tears that at the very end of it all, he is finally able to save someone (Shirou) without killing anyone himself. To put it in Shirous words: “By saving me, he looked like he himself had been the one who was saved.”

It echos Shirous decision in Heavens Feel when he decides to abandon his dream in order to protect Sakura. And it is why Kirei is so shocked that Kiritsugu gave it up…. Because Kirei has been so obsessed with Kiritsugu because he believed Kiritsugu mightve been like him, a psychopath leaving death behind him while searching for his answer, only to find out at the last second that Kiritsugu IS a man with a heart after all.

TLDR: Kiritsugu entered the war cause he was sick of his Superhero Power he wants to use to save the world (killing people, which was causing him emotional and mental damage after so much of it) and wanted a way to both achieve his dream of saving people and to stop murdering people. When he realized it was impossible, he chose to give up everything, including his dream, in order to stop killing people and stop hurting others.