You're begging the question by defining it as revenge.
The state should be in the justice business.
Justice for an intentional and premeditated killing could be seen as forfeiting your own life. It's certainly the ancient Judeo-Christian tradition.
Have to say there is something unfair and creepy about a murderer living another 20 years or more in prison, while the victim's life was snuffed out prematurely. Can certainly see how the victim's family feels it's unfair the killer gets to communicate with loved ones and go on living.
But I would limit it to depraved killings, mass murders, no doubt of guilt. First degree -- planned and intentional murders only.
The execution Trump engineered last week: a few guys talk to a couple who stopped to use a pay phone at a convenience store. They ask for a ride, with intent to rob them. Pull out a gun, force the couple into the trunk, pick up some friends, ride around for hours trying to use the couple's ATM card. Then the gang leader decides they have to kill the couple because they've seen his face. A rather tawdry pathetic crime. What was the plan?
So he shoots them in the head and orders a 16 year old in his gang to dump gasoline on the car and burn the evidence. Some evidence suggest that the husband was killed but the wife was still alive after being shot in the head and the smoke killed her. The 16 year who set fire to the car was just executed.
It seems he was planning to be part of a robbery. Someone else escalated it to murder. And he took a somewhat small and unfortunate part in it. Was also just 16 at the time, and said he feared what would happen if he didn't obey the command to torch the car with the bodies in it.
This wouldn't meet my threshold at all. He didn't intend to kill -- he thought the couple was dead. He was under some duress. He was just 16. He only intended to commit a robbery, etc. Even life in prison seems to harsh to me.
20-30 years jail time would be stiff.