I'm trying to rationalize what insanity actually is using the famous Jigsaw from the Saw series as an example.
See, while it's easy to write him off as insane/crazy because of his philosophy and antics, it's the fact he is so smart and determined with conviction that calls it into question if he's really unaware of what (the suffering) is going on around him. He's completely convinced himself he's not evil or killed anyone, which does give credibility he is pretty out there in the head. However, the level of rationality and planning he displays means he could just be believably sane and therefore completely evil.
I think whether this man qualifies as anti-villain (note: not anti-hero; a full-fledged villain with pitiable or sympathetic motives) hinges on his debatable sanity.