Liberals oppose the death penalty because they can imagine themselves receiving it. They know deep in their guts they are overstepping civilized bounds with their constant actions and beliefs and viscerally but vaguely feel they are subject to the death penalty. For complementary reasons they are all for abortion. They cannot be aborted, being already born and adult.
But seriously, I am curious as to why liberals advocate against the death penalty, when all of their other support goes to abortion to euthanasia to assisted suicide, etc. Your explanation is a decent one, because they do overstep bounds of decency and being civilized. Maybe its just because they hate western Christian, age of enlightenment, civilization and are just very bad people?