It’s not legal to kill someone who’s done something terrible when they don’t currently pose a threat to your life, deterrence be damned. Not only do we have zero evidence that the death penalty acts as an effective deterrent, but even if it did that would be irrelevant when weighed against the morality of murder.

Anyone facing the death penalty has already been successfully apprehended, convicted and jailed. They pose no threat at all.

Let’s say someone’s mother is murdered in a horrific, targeted manner. Then they find out who did it, they know their name and address, and they have concrete proof. If they kill this person instead of referring this information to the Police, they would themselves go to jail for murder.

There is no moral difference between a person committing this murder and the state using the death penalty.