What Kestrel said. Also, villains fall under a yardstick of measurable humanity versus disturbia. Going too far in either direction kills the villain element. (A villain who is too human to do evil things is an anti-hero, a villain who has no humanity is a one dimensional villain who is entirely incomprehensible to the average person.) 

There are also certain moral event horizons and special kinds of evil to watch out for. Crossing them typically puts you in the one dimensional evil territory unless you build the villain up to that level of evil. (Else he is just "generic edgy bad guy".) Think of things like murdering children, raping people, slaughtering/torturing cute animals, and so on.

The best villains are the ones who have some kind of restriction on their ability to do evil: Mentally, physically, or even something as simple as "society wants this guy dead so he has limited influence." That way there is some measurable, identifiable, relate-able justification for any extremes they might resort to in order to accomplish their goals. (ex: I want to take over the world so I can build my sky palace and have infinite women to love my pathetic ass, however I'm physically crippled, so I need to subvert others to do my bidding and sacrifice any who get too greedy for power themselves...)