Quote Originally Posted by Blodrast
No, killing enemy agents in your provinces doesn't get you to a war with them.
Your inquisitors burning their peasants doesn't get you into a state of war with them, either. More likely, they'll try to assassinate those inquisitors, but that's it ;) Let'em try
Broadly correct, but I do think that the AI is less likely to accept alliances/ceasefires when you have a lot of agents (spies, assassins, I am not sure about religious agents) in his provinces. But then, the diplomatic AI has baffled many a MTW player with its sheer randomness.