I don't think it's a very good point in the context of this thread. I think it's a bit of a straw man tbh, because I can't really see anyone saying that killing is wrong in all circumstances here.
The death penalty is useless for anything except revenge on any measure - yes, it removed the bad uns permanently but that's an issue if you eventually find out they weren't actually a bad un, or at least not in the particular circumstances you convicted them. Safer to imprison them.
It gives a really rubbish signal re: how serious society considers murder. We know that most of the people who are found innocent were done so because of inadequate investigation, misconduct on the part of law enforcement, prejudice etc, etc. If society really cared about murders facing their just desserts then they'd want to make sure they had the right person - in each case someone else is free to continue breaking the law.
Criminals show us that they agree that it's giving a really rubbish signal because they keep on killing - it's not a deterent.
It is popular though... maybe that helps to get to the bottom of why it remains on the statue books of a country where most people involved in the process are elected.