Hmm...maybe he believes they're a hindrace to the law,and so is acting like the laws that are already set down. If you (and i can't remember the technical terms in this instant) disrupt the procedures of law, you are arrested.
While, what Light is doing is severe, he may be keeping to those severes, as those people who he kills won't necessarily be on death row, especially not in other countries that dont' inflict the death penalty, and you get killed by him, no amtter what the severity of the crime is. Perhaps defiance of his idea is a crime to him, and so he believes those who are depicted as innnocent are guilty.
In a rather "twisted logic" kinda way, it makes some kinda sense. And he DID only kill 13 innocents (12 FBI + Naomi), not counting the guy L stuck on tv whose name escapes me at this time. And while placing value on someones life isn't right, it's not that many, compared to the hundreds of criminals he's killed so far.
We just htink it is, as we are actually told that all those people died, while he's still killing criminals, even while he's defending his position.
I think at the end of this series, I could write an essay on it. My posts are so looong.