Wow. I'm not sure where to start. Mistakes were made on both sides, to be honest. Let's see...
You're 18th level and you're still taking orders from kings? What's up with that? You don't take orders from kings no more. You take SUGGESTIONS. If a King has a problem with showing some decent respect to someone who could destroy and rebuild his whole castle twice in a day, he should hire lower level characters. I think you would have been justified in kicking the kings ass and making him apologize for being such a dick to his betters. Remember the Interdict, baby.
Somebody's been reading way too much KoDT.
You obey the king for the same reason that special forces, expert marksmen, and 10th-degree blackbelts follow the law in the modern world.
Because it's the law. Because that's the way society works. It doesn't matter if you're a 1st level commoner or a 20th level wizard, the king of this kingdom is the rightful ruler, and you show him respect until he does something to prove that he's your enemy.
(And BTW, this isn't a "Well, my character's chaotic, so he wouldn't do that situation." If you're mouthing off to kings, that's not chaotic, it's stupid. No matter what level you are, somebody's out there who can take you down, and if you make yourself an enemy of a king, I guarantee you he'll find them.)
And requesting that all weapons be left at the door does
not qualify. That's a standard procedure for many kings/dukes/members of royalty. It's a basic precaution, and common courtesy, if not respect for the law, should inspire obediance.
The fact that you
can kick someone's rear and level his castle doesn't give you the right to do so. Unless you're playing evil characters, and even then, it wouldn't be the smart thing to do.
That said, however, it sounds like your problems with this DM go beyond this one incident. The slapping thing was way out of line. And as far as
this goes:
all ending in a "if you dont take the money and go adventuring, then you can go home cause I aint got nothing for you to do..."
That's the mark of either an inexperienced DM, or a lazy one. And you say you've been playing this campaign for two years? Hmm... That seems to rule out option one, doesn't it?
You also say that he has to have the last word on everything, and that his important NPCs are unbeatable? I've played in games like that. In my case, the DM was a control freak who's primary purpose in gaming wasn't to tell stories, or to make sure everyone had fun. His goal was to be the "boss" and get to do things his way, no matter what.
Not saying that's the case here, since I don't know your DM, but that's what it sounds like.
Have you given any thought to switching DMs?