Because you might find out tomorrow that the person you killed was actually innocent. In contrast, if someone is freed after wrongly imprisoned for 3 years... a

Because you might find out tomorrow that the person you killed was actually innocent. In contrast, if someone is freed after wrongly imprisoned for 3 years... at least that person is now free and alive. Even if it's 1/3000 people are actually innocent, that's enough reason not to use the death penalty. And even if the person is guilty, I'm not convinced killing someone is the correct punishment for any kind of crime except for some very rare extreme situations that I can't even think of right now.