Well the whole point of capital punishment is that you're taking away the criminal's right to life - as in they no longer deserve to continue walking the earth just to cause trouble. It's not like anyone's trying to sentence them to an eternity in Hell or anything like that - what happens after you die has nothing to do with the death penalty (I'm not saying whether I support/oppose it, BTW).Jeffrey Dahmer, for instance; the man did murder, rape and cannibalize young men, but I wouldn't call him "evil". He wasn't wringing his hands and cackling while he did it, he was mentally disturbed. He killed those men out of a twisted display of love, but that's not evil. It wasn't done maliciously. The man's wires were clearly crossed, and he didn't deserve to be beaten to death in prison. It's reasons like that I disagree with the death penalty.
Because when you think about it, what is death exactly? For all we know, maybe it's an existence greater than life itself, when you finally become one with the universe and gain an understanding beyond normal human comprehension. There's no reason or evidence for suggesting this, but then there isn't any for a divine puppet master who hates them gay folk, either.
Under Kira's plan, you can supposedly stay away from his "justice" by being a 'good' person, not to mention staying out of his way. But your example seems pretty random - I don't know if anyone would agree with it.There's no set meaning for "justice"; like good and evil, it changes. I could instigate genocide under the pretence of justice to prevent overpopulation, but would you agree to that? If not, then why the hell do you agree with the ambitions of a narcisstic little **** with a Messiah complex?
All that said, I do agree that Kira was not justice, because he (nor any human) truly knows or fully understands what justice is. Honestly, Kira ended up being the insane one himself - he even literally went a little mental in the final episode, when his plan finally broke down on him.