Sure, it's acceptable. But not because you're angry with him. If you're angry with him, dealing with that by punishing his character is going to just make a bad game worse.

I kind of can't believe that you guys are advocating for dealing with player-to-player frustration by taking it out on the character. Like I said, that's passive-aggressiveness, and it's the kind of stuff we did when we were adolescents, right? But not anymore.

an in-game reaction is completely acceptable. Again, sure, if that's the kind of game you're playing, and you're all cool with backstabbing. But that's not what's going on here. The guy is being a complete douchebag and ruining other people's fun. The solution here is to pause the game and say, "Hey dude, that's not cool. That totally ruins my fun here. That's not the way we play," not to be a douchebag back to him.