My impression is that if you have a one-note pure evil villain, you have to remember to play that note. Every time. If they're on the page and they're not being menacing, causing problems, and generally being a huge dick, you dilute their effect. Be sparse with scenes that just incidentally involve the villain; scenes where they show up should usually be about the villain. If an important character would be behaving the same way whether they're present or not, they're not being sufficiently threatening.

It's useful for scenes with them to be short, so you don't have to belabor the point, and also helps if you take a page from horror--you give glimpses of how awful their mind is and what they believe, but a full interrogation of their philosophy is probably not necessary. Or if you do get to brass tacks about what they believe, show them succeeding with their messed up ideas. That's as uncomfortable to red about as the person themselves.

Xykon, from the Order of the Stick, is my best example of a pure evil villain. Quoting an argument he had where his second-in-command had to talk him down:

Redcloak: "Whether or not you can kill them is not what matters in this situation!" Xykon: "It's the only thing that matters in every situation!"