There's a couple different ways I enjoy it, though it's never a shock value thing. From the murderer pov, it's definitely a power/thrill thing, completely controlling a person and doing what I want with them.

The victim pov is more difficult to explain. Thinking about the death process, slowly dying (violently or not) seems almost... comforting or peaceful? Maybe cathartic? But it also has some elements of the thrill that I mentioned earlier. I would honestly tie it to suicidal thoughts if I hadn't been experiencing this one for my whole life, way before I developed depression.

The weirdest thing I guess is that I probably wouldn't enjoy revenge murder at all. I only fantasize about people or characters I'm quite fond of.