As many have posted previously, it’s weird that people cheer on Vader slaughtering a bunch of frightened rebels who are clearly no match for him. It’s like people cheering on an authoritarian government enforcer gunning down innocent civilians.
I think you misunderstand people’s reaction to this scene. People like it because it shows a villain being very villainous. He’s intimidating, scary, and brutal. It’s the same kind of gut reaction people have to any “good” villain. People don’t “cheer” for the Joker, but they love it when he’s frightening and a menacing adversary for the hero. It’s fantasy. The worse a villain is the better the heroes get to be. Enjoying a good “bad guy being bad” scene is definitely not the same as cheering on Stalin or Mussolini. I find it difficult to grasp how some Star Wars fans don’t understand the appeal of a really vile villain.
Also, it’s an exciting action scene with lots of cool force powers and stuff. People don’t cheer for Voldemort but they sure like watching him use all his magic. It’s a spectacle.
You could also relate it to watching a slasher film. There’s a LOOOOOOONNNGGGGG history of people enjoying watching/reading terrible things in their entertainment. I’m sure there’s some deep psychological explanations but the bottom line is people have always been entertained by awful things.