The reason Kenobi didn’t kill Vader is because he feels sorry for turning Anakin into this monstrosity, so he doesn’t want to kill him. He’s this monster with the face of someone he used to know and care about.

Like Luke Skywalker, he just couldn’t kill him, even if it’s the right thing to do. Underneath all that armor he’s still Anakin Skywalker.

After spending a few days with Anakin’s daughter Leia, he doesn’t want to be the one to kill her real father and have her resent him for it.

Maybe Kenobi was hoping Anakin would one day turn away from the dark side, kill the Emperor and redeem himself.