Again, being humane in our execution is not for the convicted person, it is for us. IMO searching for a "humane" way to kill a human is an oxymoron. A health, normal person feels sympathy for a human going through distress; they project themselves into the person's situation and can feel their pain. It is why we cringe when we see someone being hurt, or we look away. This sympathetic feeling is what we acknowledge now and try to reduce as much as possible. Unless you get a high off of vicariously feeling the pain of others, then you'd likely want to reduce the pain you cause others through life. This is one area we can certainly do that. As humans it is impossible to decouple our sympathetic feelings from a person due to what they have done. As in, we still put ourselves in their position and feel their pain even if they are bad people.
Saying "hell yeah, he got what he deserved" is most likely a way for you to avoid investigating these sympathetic feelings. It is a western ideal to bury them.