It’s important not to conflate moral facts with practical policy. Most of your arguments focus on how people should be treated, whereas the relevant question is how governments should behave and why. These are very different things.
Regardless of what people deserve, no government should go around killing its own citizens. That is because killing as a punishment makes a spectacle of death. It is profoundly unhealthy for any civil society to revel in death. That’s the answer. It has nothing to do with what serial killers deserve. They do not matter.