Personally I think it fully depends on the circumstance. Its not a one size fits all situation.
Victims of domestic abuse finally snapping and striking is different - in self defence. Getting into a fight and accidentally killing someone without the intent is different. But the people which I would fully support facing the death penalty are those who deliberately set out to cause and torture on another individual. Like that nutter in Colerado for example, he knew exactly what he was doing and he set out to kill those people deliberately.
I actually he is ill, so whilst he may look like he knew what he was doing, I dont think it would be right to give someone with mental health problems the death penalty.
But then where do you draw the line between being 'mentally ill' or just plain cruel and evil?
Generally the line is drawn at the diagnosis of mental illness...
Is there such a thing as mental illness in these cases though? Maybe. Maybe in some cases its just an excuse or a desperate case of reason.
If there is, what about those who are wrongly diagnosed? I don't know much about the tests done to be honest, but what if they're not ill and they're mis diagnosed but they get off lightly for it?
Theres too many what ifs surrounding this topic. It is very interesting though.