The Anti-Villain is the opposite of an Anti-Hero - a villain with heroic goals, personality traits, and/or virtues. Their desired ends are mostly good, but their means of getting there are evil. Alternatively, their desired ends are evil, but they are far more ethical or moral than most villains and they thus use fairly benign means to achieve it, and can be rather heroic on occasion.
They often reach a kind of critical mass that makes them more good than normal villains but not quite heroes, blurring the line between hero and villain the same way an Anti-Hero does, but by coming from the opposite direction. Also, they seldom perform a full Heel-Face Turn, or if they do, they continue to struggle with their villainous impulses. Anti-Villain is an attempt to humanize, to lighten up, a villain as opposed to an Anti-Hero, which has a tendency to darken the hero. Side by side, it can become very hard to tell them apart. The only reason some would even be considered evil at all is because they're the Designated Villain. Despite this humanizing characterization, they are rarely less dangerous. For instance, heroes wouldn't know what to expect if their enemy shows caring and then attacks their reputation, without giving them an excuse to rationalize killing them.
Most of them are probably well aware that what they're doing is "evil", unlike the blinded Knight Templar, but strive to maintain a facade of good PR. They'll see it as a viable means to a (possibly) good end.