Yes, the only real argument against the 100% cases that I've ever heard is that there would be fewer people admitting to murder if they knew that they would be

Yes, the only real argument against the 100% cases that I've ever heard is that there would be fewer people admitting to murder if they knew that they would be killed. But I think this shouldn't bother us, so what if fewer people confess? I think most confessions are already only done when they see themselves having no other choice than admitting what they did. Also we are talking about the 100% cases, people who e.g publically shot someone or are so mentally ill or stupid that they confess anyway where by definition we might not even need to wait for a confession. I think most people who are against the death penalty are not fully honest with their opposition, consciously or subconsciously. Outwardly they say that they oppose it due to the danger of innocents being hurt. But inwardly I suspect a far less well intentioned reason because we see an indication for this in the release of murderers from prison after only ~15 years sometimes. What genuinely moral person would ever support the release of a murderer? But see, if we don't execute murderers immediately it starts a whole process in humans. They see the suffering of the murderer in prison and they feel bad. It's automatic, they can't turn it off. This the kind of automatic biological pity when you see someone suffering which is not a good kind of pity in this case. This person murdered another human being, his life should be forfeit. But then an even sicker consideration starts coming into play: The humans who see this other human (murderer) suffering say: Well, this is a waste! This too is inevitable thinking in humans. We see a human being who seems to behave well now, in a cage, and seems to be nice and you can talk normally to them over topics like the weather and family and suddenly the thought comes that this is an economic waste. Then society suddenly has a big, daily economic incentive to release him. This is why I support the death penalty to be carried out immediately, we cannot trust people to be objective about pain and suffering, especially not decades after the committed murder when the wounds aren't fresh anymore. But the victim cannot talk and plead for justice.