When do you think its approriate for a criminal to be killed?

I recently watched something really disturbing on tv about this couple of killers who had fantasies of killing children so what they did was lure children over, torture them for hours then skin and burn them alive. and while they did this they recorded the screams of their victims to listen to later and they only got a few years and escaped. That really honestly infuriates me. If I had those 2 they would have been dead in a heartbeat but they got away. So when do you think its approriate for criminals to be killed. this is my look heres an ex. guy kills bunch of people for fun gets put away gets out for good behaviour does it again with no remorse. I'm sorry people like that in my opinion deserve to die. any thoughts??