Either way, you kind of just end the series with an old man’s face getting melted.
To be fair, the old man who was getting melted was the most evil person in the galaxy, so I don’t have much sympathy for him.
In general, I’m kind of glad TROS chose to eschew the whole “kill him and you’ll be just like him” trope. I get that it’s “part of the fabric of Star Wars” or whatever, but I’ve always thought it was stupid: Luke kills millions of underpaid, abused foot soldiers while everyone around him cheers, but the instant he meets someone who committed mass genocide it somehow becomes wrong to kill him? Despite all of TROS’s flaws, JJ realizing that idea is stupid is probably the best decision he ever made.
It’s not stupid. It’s because Luke would be acting in anger and doing exactly what the Emperor wanted, turning to the dark side in the process. The Force and the dark side have supernatural influence over the situation in a way that they don’t in a normal battle scene. The conflict is a primarily spiritual one and has nothing to do with the outcome of the Battle of Endor. “Soon I’ll be dead, and you with me.” That’s one of the Emperor’s temptations, to goad Luke into thinking he can reverse the losing battle, or take revenge, if he just works up the nerve to kill him.
Luke wasn’t going there to take out the enemy military leader or something like that (such as Obi Wan vs. Grievous.) If he was, it would be a sticky ethical question like it is at multiple times in Revenge of the Sith (such as executing Count Dooku and Mace Windu getting ready to execute Palpatine.) But that’s just not why Luke was there, it wasn’t about that.
Since the Empire is already built and the damage is already done, Palpatine is really only an old man who can shoot lightning bolts (and none of the rebels knew the second part.) It’s not Luke’s responsibility to carry out punishment. The Death Star and the Emperor are going to get blown up anyway, and he’s on a personal quest to confront/redeem his father.