This is a leaked video from Blackwater in Iraq. It's some horrifying shit - they're just driving into town running over people and shooting random people for fun. Obviously the content is shocking, so careful if you're not ready to watch this. Also, imagine the shit they did that wasn't filmed/leaked.
Because they're not negotiable, to me. You start to exclude certain people from such rights, and pretty soon there's a risk such exclusions are enlarged to other group of people.
But that's basically the slippery slope argument of course; yet it's not the only reason to me. Many of the child murderers you talk about, for example, while fully responsible, arrived at their actions due to truly fucked up pasts. Society often does have a measure of responsibility for them ending up this way.