What's to stop a Justicar from just intervening?

Let's say, for the sake of argument, that the Prince of a major city has decided that he doesn't really give a shit about upholding the Nine Tenets anymore. Let's also say, for the sake of argument, that this particular Prince is a strongman who's consolidated his power and essentially prevented anyone in his city from holding him accountable.

This is the current world state of the chronicle I'm running. So with all this being said, with the Masquerade worn thin and the Second Inquisition circling overhead, what's to stop a Justicar from just rolling into town, lopping off the Prince's head, and being done with it? It seems like the most logical course of action from the Ivory Tower to me, but this Prince has been kind of set up as the major antagonist of the story and I don't necessarily want to rob my players of their revenge.

Any ideas? I'm open to suggestions!