If A is bored by B's play, and therefore takes steps to make things interesting, without knowing more I have to treat it as an open question whether it's the proactive A or the boring B who is "in the wrong". But in any event that's not what I saw in the OP: a player, presuably playing his PC is frustrated by the Mad Tyrant and verbalises that frustration, then the GM narrates more stuff which includes an escalation to violence, and then another player has his PC respond in kind:
{A PC] yell[ed] out something to the effect of "you're crazy and don't deserve leadership here." For this affront, the ruler yelled for his guards to come and arrest that character. In response, another party member tried (and failed) to grapple the ruler and put a knife to his throat
That's the GM, not any player, who treated violence as the universal solvent.
First, as with
@chaochou I'm curious about what the resolution method is that was used to make the move from
PC yells out to
Mad Tyrant takes it badly to
Mad Tyrant calls for guards to
guards arrive and follow his order to arrest PC. At every point I can see a different possibility: the Mad Tyrant laughs off the insult; the Mad Tyrant personally challenges the affronting PC to a duel of honour; the guards are all drunk and don't come when called; the guard captain agrees with the PC that the Mad Tyrant doesn't deserve to rule, and seeing now a chance to strike against the tyrant takes up that chance. And I came up with those possibilities in the time it took me to type them up.
Second, the Mad Tyrant isn't a natural phenomenon. It's an element of the fiction in a RPG, presumably intended to serve some purpose for RPGing. What's that purpose? To create a puzzle for the players, where if they don't guess the right thing their PCs die? (This is
@Nagol's analogy to the trapped door.) To allow the GM to tell the players what their PCs should do on pain of dying? A chance for the players to show off their ability to follow the GM's lead? Something else?