From a guy that is more right than left, you really missed the point. Of course he is guilty beyond doubt. You only have to look at the countries that still have a death sentence to see what group the US associates this with. That after ignoring innocent people and costs etc
Killing while necessary for self defense can be justified. The death sentence is like killing someone who is running away from you and no threat. It is barbaric. I don’t think rehabilitation is in anyone’s thought. Personally for life sentences I don’t think the prison system has to consider that at all. I don’t think it has to be particularly cruel but I don’t think it has to be particular comfortable either for those who can never join society again.