Tuura speaks well. I'll address another angle further.
As Tuura points out, the distinction is between player's whim that doesn't fit the character, and an actually sociopathic character. I'll assume that the first is the case, since that is the trickier situation -- all the medicine on the table so far is eminently suited to dealing with the second.
The question is, what do you want? Get rid of the player without explicitly kicking him out, or make him mend his ways? In the first case, punishing the player is exactly the way to go, but in the second I'd recommend against it.
Assuming you want to make him to (1) never do it again and (2) understand why it was not OK, you need to find out why he did it. Question is, does he know he did wrong, and does he know why it was wrong? People can be both astonishingly stupid and incredibly astute at times, so if he was my player I'd just ask, even if I was sure I knew the situation.
If he was aware that what he did was not acceptable, and is one the same page with the actual reason for it not being acceptable, then "WHY THE HELL DID YOU DO IT THEN, YOU TWIT?" would be my next question, followed by rescinding his invitation to the game. ,-)
If there was some misunderstanding on his part, then I'd explain the situation to him from my perspective. If he doesn't get the message (or gets it but doesn't care), then kicking him out is the only reasonable action in my book. Most people (in my assumption), however, are not likely to engage in deliberately disruptive behaviour: either they mend their ways easily enough when they realize what the shared goal was supposed to be, or they realize that this is not a game they actually want to play.
Of course, if he is a buddy outside the gaming context also, then kicking him out can be socially awkward. The best you can hope for in that case is probably to get him to "resign" himself, saving face, etc. AKA "Not My Kind of Game Guys, I Hope You Don't Mind" solution.
(Obviously, I'd not hold this discussion in front of others, but privately.)
As for in-game repercussions, I'd talk to him about them after resolving the possible misunderstandings. If he was under some wierd misapprehension, and it really seems that he would not have done this had he understood what kind of game you are trying to play, then I would try to minimize the situation. Maybe he was under some sort of spell from a malignant fairy-lord, who wanted to get one of the books from the covenent? Whatever. Water under the bridge, etc, provided the player is less of a problem in future.
Out of curiosity, what did the other players think of his escapade?
Not Ars, but sort of on-topic still:
Robin's Laws of Gamemastering
has lots of good, sensible, down-to-earth stuff about communication between players and gamemaster (and more).
As a secondary point, I sometimes play "troublesome" characters whose actions should cause them a world of hurt, and every time a GM is lenient and lets the consequences slide, I am deeply frustrated: I wanted the consequences! I wanted to get the character into deep trouble! ...so I'd say that if this was all in character after all (the character just happens to be a murdering SoB), then toasting him is what I'd want, if I was the player...