Earlier quoted context omitted.
Why? Because it is "legal". Do you base your morality on what is legal and what is illegal? How do you define murder?
Sincere question. Why are you allowed to murder someone in self-defense, but not as a punishment for crimes? What moral code do you rely on that draws the line there? Why is that moral code more correct than one that draws the line between punishment and capricious violence? FWIW, I'm somewhere between you and OP in my thoughts. Definitely against the death penalty in practice , and somewhat ambivalent in theory. But…
The principle of Self Ownership, which is the basis of the philosophy of liberty
https://www.youtube.com/watch?v=M9srplWe_QQ
It would not be murder to protect your own life from the aggression of others. However any use of violence must be in proportion to the threat to your life. I.e I do not believe you have the right to kill someone for trespassing, but only in those circumstance where there is a reasonable believe your life to be direct and immediate danger.