Evil INTP: what about the Failed Genius? The depressed, seclused one who thinks the world would never appreciate his geniality? Either he can take revenge on the world like Durentu describes. He wouldn't do it following a plan - that's for the INTJs - but he can be quite destructive, being smart and random.
Or he can be quite harmless to the world, but a pain in the ass to the main character.
Maybe that's the key to get to an Evil INFP. Firstly let's destroy a myth - Fi isn't always good. Corrupted personal values, an emotional thirst and even love can be quite destructive. Now take an unbalanced INFP... an emotional leech, randomly going from one extreme mood to the other, when he's there everybody walks on eggs and when he's away everybody is worried he might kill himself.
This one needs help and might be seen as a victim rather than as a villain. But in the meantime he's destroying the lives of everybody around him. And the people are ashamed of seeing him as the pain in the ass he IS, because, well, poor guy!
(Hm... this idea is spawning a story in my mind...)
Edit: I've mulled this around in my head and thought up some generalities about evil Feelers...
They can use their Feeler tact and subtlety as a weapon. They use emotional blackmail, know the precise words to hurt people (just like a good Feeler knows how to support people), are the popular kid in the playground whimsically deciding who's in the in-group and who not,... They don't stick out as villains. People hate them but can't find a reason why. On the surface of things, they don't do anything wrong. They get under the skin of the main character, making Main appear as the villain!