>Villain is just a cackling madman who's evil for no reason
Why is he so compelling anyway?

Because he's based

>>213076111 (OP)
Because doing evil for the sake of evil is the only true way to be a villain. Otherwise you're just a misunderstood normal person not an actual villain.

>>213076111 (OP)
what does he think about when taking a long shit?

The Emperor, in the original trilogy specifically, is meant to be more of an embodiment of the dark side and the empire than he is an actual character. He's compelling because he's played charismatically, he looks cool and he's the perfect anti-thesis to Luke while also being the perfect foil to Vader. Illustrating the end result of both character's journeys if they give in to the temptation of the dark side.

>>213076111 (OP)
Kino performance, he's like a sassy old gay guy but also scary somehow