Killing someone for what he has done in the past is not punishing him, it is taking him the possibility to show that his heart, his humanity is not dead yet.
Not killing someone who as a lifelong penalty is a tribute to his live, a possibility for him to become human again.

You are not a mass murder, you do not know what drive those. Are we really able to judge people like them? Of course the compassion for what those people did to lovely people can easily become hate, but I cannot help to feel sorry for those people. Sorry because they somewhat became misguided humans.
Death penalty is just the most arrogant, narcisstic thing human beings ever came up with.