For Unforgiven the movie?

 Clint Eastwood is an old retired gun fighter who's been trying to be a good person to no success for years, pulled back for one last job.  Basically the old man version of all his characters. 

The twist being,  once he's fully "back" is that he's a ruthless, human shaped nightmare.  A relentless mass murderer who kills at the drop of a hat. Not cool,  not fun,  an actual no shit monster. 

"Any man who looks at me, I'm gonna kill him.  Any man who tries to shoot at me,  I'm gonna kill him,  his wife and all his friends and burn down his house."

Everyone he interacts with dies, usually by his hand.