How would you make existing supervillains scary?

Ok so to elaborate, I feel like a villain should be someone that wants to hurt innocent people. Like yeah some villains are "right", but in my opinion a true villain is someone who genuinely wants to hurt innocent people for whatever reason. It can be a good reason or no reason, so long as they enjoy doing it.

Like Michael Myers or Freddy Krueger. Sure they're not someone like Dr DOOM or reverse flash in terms of overall villainy, but it's the terror and fear that they represent is what I want.

So how would you make supervillains (I'm talking genuine bad characters) scary?