I think that the question you have to ask yourself then is whether it is worth removing the terrible people if the cost is executing innocent people (as has happened in at least one case in Texas and, I’m sure, elsewhere).
Morally it can be a tough call. Is it more human to execute someone or lock them up for life in a place like California’s Pelican Bay maximum security prison?
My take on the issue is that the death penalty degrades society. But then we also kill people in war (drone strikes, air strikes and other military action). So it’s difficult to draw a clear and consistent moral line.