An entirely reasonable statement. It's quite hard to disagree with, in and of itself. However, I think that it's worth noting that the reasons why one might find the death penalty acceptable aren't limited to just that. To poke an easy example, if the (perceived) risk associated with not simply ending the lives of those found to be sufficiently bad offenders is greater to innocent people than the risk associated with accidentally ending the life of an innocent person, that would make for a pretty direct counter to the principle you invoked. After all, those people are being executed because more harm to innocent lives would normally end up occurring than if they weren't, not because some nebulous "deserve" was invoked.
To be clear, I'm not making an argument for that position, specifically. Rather, this is one of the subjects where it's a bit too easy to effectively retreat to something like the claim that innocent lives are sacrosanct, which is something that I do take issue with. The moment that something is declared sacrosanct is pretty much the moment that one is declaring that they're simply unwilling to properly deal with the subject and leaves opening for practical absurdity.