I think Matthew Holm probably hit the nail on the head, but I would also argue that a character can be “just bad” without being one dimensional.
Plenty of movie villains have tragic backstories to explain their misdeeds but also act inconsistently with no understandable motivations.
I don’t generally care why a villain decided to be a villain. I care why they’re doing this particular bad thing. Darth Vader had consistent motivations and I felt like I understood who he was long before I ever saw him clashing against the rules of the Jedi Order.