I can continue. The world is messy. Black white thinking will not do it justice.
Not that I agree with these reasons in all circumstances. I just want to point out there are reasons to treat a murderer less harsh.
If that sounds irrational to you: That's because it is irrational. Whether we choose to end another persons life is fundamental in our understanding of life itself, which is highly subjective.
But your belief that we should kill other humans is also irrational and subjective.
Edit: Also this paragraph in your first answer
>> I also think that pain should be part of the death penalty... that a murderer doesn’t deserve a quick and painless death but rather a healthy period of searing agony to experience the anguish that they themselves wrought on another."
is actually terrifying to me. Even reading it causes me some physical discomfort.