The world population is already predicted to reach 9 billion by 2050, there will eventually be a population problem if there isn't already. Logically we shouldn't waste resources on downright evil people like this..
The problem with the death penalty is it costs just as much as keeping them in prison for life when you consider all the appeals, which are needed to stop innocent people being put to death.
I'm only against the death penalty as long as there isn't a way to guarantee that everyone put to death is 100% guilty. Which I doubt will ever happen.