Like most commenters here I dislike villians that are "in your face evil". But that's too easy of an answer to me already. I have to elaborate:
In my opinion even villians have to be characters who can be identified with. Which does not mean that their stand point has to be morally in line with me. He has to have a cause which is based upon all three terms of time: Past, present and future. There has to be a moment in the past that made him what he is now, leading him to the ultimate goal.
A bank robber can be a good example. Maybe this villian has a little family but he struggles to get work because he lives in the wrong neighbourhood. Companies don't take him although he has a good record, but because of his adress they don't want to take the risk in hiring him. Thus he is forced to take immoral measures in order to feed his family.
This way one might sympathise with this villian, even though his actions are still wrong. Maybe he even takes hostages after he managed to get into the bank but police was faster than him. Then he is also a kidnapper, which makes it even worse. Or he accidentally shot a security guard because he was to nervous with his gun ect. This all though does not lower his plausibility.
Scenarios can be layed out up to the biggest evil one can imagine. Like a King who became an "murderous tyrant" but in reality only became like this because he was forced to become king of this land and was not able to marry his love because she was a low born.
Thus said: Even villians deserve a background and empathy. They may even be people who KNOW doing what they do is wrong (like the robber), but see no other way out of their misery. Or at least have to think that what they do is for a good cause, no matter if it conflicts with our understanding of it.
Ah yeah and then ther is always insanity which is a good trigger too, but can be overused as well (like say: Joker from DC).