Well, the thing about Kira is that he wanted to become God, so the term criminal is very loosely defined in this case. Some people do deserve to die, but in some cases, whether or not someone deserves to die is very vague. I guess the question is if you want to leave such an important decision up to one person?
Exactly. Kira's God complex was what brought him down in the end. Were he doing it because he truly believed it was right, that might have changed things, but it was if he more wanted to know he could just do one little thing like writing a person's name in a notebook, and they would die.
Also toward the end he seemed to just kill anyone who got in his way or disagreed, even those who were doing it for their own justice (like L).
So his general idea of killing criminals, that I support. Doing it in twisted ways or without fair evidence (I believed he killed some men in the first season just because they were harassing a woman?), I do not.
Kira didn't consider the consequences, no matter how clever he was.
