Well, let's just take Nao's case for a second.
She grew up with terrible, terrible parents, mkay? Her parents left her, mkay? She then finds a beautiful husband who loves her beyond everything, bears two of his children and even gets accepted by his parents so much so that she even holds them as her own, mkay? She then promises to herself that she'll one day repay them all as best she can for the beautiful life they've all given her.
.......then she proceeds to kill them one by one, no no no no, even worse - turn them into Shiki; a fate worse than death. (nevermind, she just killed them)
Yeah, I think I'll show her some compassion by pouring her with molten metal and then crucifying her publicly.
As I said earlier, their logic is utterly one-sided. You know, just because you have the desire to live, doesn't give you the right to take other people's lives. I know I sound like a hypocrite, as we are all, since we eat other life forms without remorse, but they aren't on the same intelligence rank as we are, thus making it somewhat of an invalid point to use against me.
They have no right to torment other people with something that is much worse than death itself, just because they got caught up in it. That logic is unreasonable and can't be taken into consideration when working towards any kind of sympathetic solution, so no, fuck the Shiki, they all ought to die for what they've done. Let's not even start with the fact that they've embraced killing like a casual sport.