Death Note
SPOLIERS: Death Note is one of those shows that makes a person confront their morals. Light discovered that book and his first use of it was to save a group of kids from a deranged shooter. Good things. But as the show progressed the power overcame him, as you'd expect from any human being. A god complex formed and the killings became more of a game to him; he was playing and mouse with the lives of even his most loved. Gradually of course he lost his mind, he died a deserving death. But his transformation from a high-school kid to a cold blooded murderer wasn't overnight.
We can all agree the killing of that shooter was morally correct, or maybe we can't, but nonetheless the majority of people can agree it was a necessary evil. So, the viewer sides with Light. But with every kill, they become increasingly more malicious and un-moral, by the end Light has lost the support of nearly every viewer: The point at which you realise he is wrong speaks a lot about you, how you value human life and what can be deemed necessary for good.
For me, it was the death of his father. Maybe that was too late and me myself value life immensely less than I should but that in itself was confronting. This is all fiction of coarse and easy to brush off but when you truly learn how you value life itself, you begin to realise what kind of man you are... And I'm still processing my answer.