That's an oxymoron.
If a killing is legal, it's not murder.
Murder is a very subjective term. I, personally, consider most capital punishment in the USA to be unjustified killing. But it's legal, so it isn't murder.
Tom
You are right, as to the applicable words.
Yet, the premeditated killing of another person in common law is murder.
I once, as a young man, was unsure about capital punishment. Then, for a short period of time I had a man who had just killed three people, in my custody. The day before, after being paroled for 2nd degree murder, he hijacked a car from two women, obtained a gun, and wound up in my county.
He killed in absolute cold blood two police officers and a teenage boy.
I saw the pain and anguish of the family members at the hospital where one victim died, and the wounded killer was being treated.
The suspect had dead eyes like a shark, and showed absolutely no remorse for what he had done.
Capital punishment deters 100% a murderer from killing again.
The life of an innocent is worth more than all the lives of first degree murderers combined.
If that suspect had given me reason to shoot him, the world would be a much better place at that instant.