What if Luke managed to push Darth Vader into the carbonite freezer machine and the rebels captured a frozen Darth Vader! Would they use him as a bargaining chip, unfreeze him for information/forced cooperation, or kill him?
shareI recently replied to a similar "what if" about what if the rebels had captured the Death Star and taken it over somehow? I imagine it would have become a huge political issue and a target for recapture by the Empire or other forces if it still existed. It would have been the cause of ongoing issues in the galaxy for sure.
So I think a captured Darth Vader would produce a similar problem.
Better to put him through a trial and execution for his crimes against the galaxy / humanity, in the same vein as Saddam Hussein was.