@Michael: Of course, I'm not making the case for actions without consequences. I'm making the case for actions having interesting consequences because consequences that aren't interesting to everyone aren't worth our precious entertainment time. Interesting is in the eye of the beholder and, when in doubt, the players should be consulted to be sure the scene is heading in a direction everyone finds fun. What we see all too often, especially in games like D&D, are DMs that unilaterally hand out punitive, boring consequences (town guard much?) for actions they deem stupid. This is a form of improvisational blocking that can hurt the game by burning trust in the group. The action the player is taking that the DM deems worthy of "logical consequences" may also be a form of improvisational blocking. This is how things move toward the adversarial player-DM dynamic that is so common. This is why groups who understand the rules of improv ("Yes, and..." is its foundation) don't have these issues. How a good GM would handle this in my opinion would depend a lot on context, so I'll have to make some assumptions about the situation in my response: A fact has been established in the fiction - nobles have the right of summary judgment. The player states that his character, Ragnar, wishes to ignore it and attacks the king. If this seems out of place for the scene, for Ragnar, or contrary to previous agreements the group has made as to the tone or content of the game, the game is paused and an out-of-game conversation ensues to determine the player's real intent and to clarify the situation. Much of the time, the player is acting out because of pacing issues especially as it relates to playing out scenes that lack tension or a dramatic question. In other words, the game is boring and the player wants something to happen, but doesn't want to or know how to express that directly. (I think we've all been there as players and can empathize, right?) This is something the DM can actually fix, right then and there, by offering to increase the action and asking for player input as to how they would see that working (with or without offing the king). The rest of the time it's a personality issue which may or may not be resolved because the player isn't a fit for the group. I'm sure you know what to do in this case. If by some miracle it's determined that Ragnar really does have beef with the king, makes perfect sense in context, and doesn't run contrary to the group's pre-game agreements, then Ragnar kills the king (or maybe there's a combat). What's a good consequence for that? Your mind races immediately to his plate-mailed and eager royal guards, right? Well what if instead, I chose to Reveal an Unwelcome Truth : "The king is run through and dies, turning back to its original form... a doppelganger! The guards and assembled nobility stare in stunned silence. What do you do?" Now that's an interesting scene and it suggests adventures that follow. So, back to tips for the OP:
Handle out-of-game issues outside the context of the game. Handle in-game issues in-game, and use your players' ideas for inspiration so that consequences for actions and dice rolls are fun for everyone.
Don't block player ideas. As long as they themselves are not blocking, then their idea is just as true and valid as yours. Use your imagination to figure out a way to say "yes, and" not "yes, but" or "no." "Yes, and..." generates fiction; the other two block it. (Players: Don't block GM ideas either. RPGs are improvised collaborations operating in a framework of mechanics.)