Murderhobos or murderHEROES? DMs & PCs need same expectations

You don't have to watch the new posts on Reddit's /r/DnD for very long before you see a story of a party that got killed or thrown in the dungeon or something because they started a fight in town. The DM often asks something like, "Am I good?"
If everybody had fun, then yes, you're good. If the players are so salty that tired horses are wandering down the stairs and into the basement to lick them, then you have a problem.
Sometimes, that problem is a difference in expectations between players and DMs. Players might not understand that there's a focus on consequences in D&D, and DMs might not understand that they're provoking the players.

Case study: Tom Petty was right

Sometimes the waiting is the hardest part. DM /u/Enddar tells the story on Reddit of how their party was basically started a fight in a waiting room and was killed. They adventurers were on their way to see the king, and a captain acting as their guide was admitted, but a mage told the players they'd have to wait.

…This is when things went completely off the rails.
The barbarian tried to intimidate the mage into letting them in, but the mage wasn't having any of her foolishness. He cast Grease on her as a harmless way of saying "Don't f*ck with me." The paladin then took the spell as an attack to an ally and initiatives were rolled.
Battle ensued, guards were summoned, and the Captain was outraged the party attacked a member of the court. He ordered the guards to escort the party out of the city and that he would personally make sure that orders were given to officially ban the party from the city.
As the party was being escorted out, the Rogue managed to sneak away. As the captain was helping the mage to his feet apologizing profusely the Rogue took aim and crit the Captain with a poisoned arrow.
"If the captain is dead, he can't banish us right?"
Then the Rogue had another idea.
"I'm going to use my Levitate spell and just go up and speak directly with the king."
She proceeds to do so, and I ask, "How do you make your approach?"
"I want to make sure they see me coming so they know I'm not a threat."
"Okay, reflex save please."
"What?"
"A royal mage has spotted you approaching the inner sanctum of the king. He is taking aim at you with a lightning bolt. Reflex save please."
Needless to say, our Rogue fails spectacularly and the mage fries her mid air. She was dead before she hit the ground.
The lightning strike is easily seen throughout the entire city and the party, now outside, see their friend fall from the sky to her doom.
Surprisingly, the one who snaps is the party's mage! She casts Levitate and Vanish, crosses the city's walls, and runs to the smoking remains of her Rogue buddy. She then proceeds to go full on flaming vengeance mode attacking anyone she sees, burning everything in sight. So now she's in the dungeons.
So now the question I have is, how did things go so wrong?

To recap, one player tried to threaten a mage to avoid waiting, the mage retaliates with a (physically) harmless spell, the player attacks and from there, the party is waging all-out war as the DM tries desperately to pump the brakes. To use tabletop parlance, they went full murderhobo.

Or are they murderheroes?

Fellow DM /u/lazorwulf responds, "I think this falls under a theory I'm working on titled, 'By design, PCs are tiny egotistical warmachines.' As soon as somebody acts a little rude or suspicious toward them, get ready for a fight to the death.
"That's the moral algebra we're all used to in storytelling. If you're watching a sitcom and somebody's a little testy with the main characters, it's so the audience doesn't feel bad when they're hit by a bus."

TV vs D&D

The social rules in most D&D adventures are a little different than TV rules, though, and that can cause trouble for beginners. One of the big advantages D&D has over video games is that players can do anything — but those actions don't matter much if there aren't consequences.
It's like the beginning of Last Man on Earth where Phil believes he's the last person alive. He's stolen artifacts from museums and the White House, he opens doors with a handgun and he blows up cars just for the hell of it. But since there's no one else around and no way he can get in trouble, it's all empty.

(The trailer doesn't quite convey just how despondent he gets.)
So while "he slapped me, so I shot him in the face" might fly in an ultra-compressed, ultra-simplified storytelling format like a 22-minute TV episode, most DMs won't let that fly.

Consequences ensue

It's the DM's responsibility, after all, to try to follow player action with consequences and thereby create a world that makes sense. And it doesn't make sense that a king would grant audience to a group of strangers that started a fight in the courtyard.
So then where did /u/Enddar go wrong? Whether you think anything went wrong at all depends on what kind of game you'd like to run, but I'd argue that things went wrong when the NPC insulted the PC.

Slapping the world's most important people

At its core, D&D is a game where players take on the roles of heroes with extraordinary abilities. Players want to get out of the humdrum of their daily lives, where they're abused by bureaucrats and harassed by strangers. So regardless of how realistic you'd like things to be, your game world will sneak away from the real world and toward the fantasy "that's what I should have said" world people build with their friends when they're blowing off steam.
So in-game, player characters may all be poor antisocial orphans, but from a storytelling standpoint they are, by definition, the most important people in the universe. And even if you have absolute control over every other aspect of the universe, you don't slap VIPs like that in the face and expect them to turn the other cheek.
If they do turn the other cheek, great for them! Award them a point of inspiration! Because they need some sort of reward for being made to feel small in a game they're playing to see what it feels like to save the world by crushing monsters with meteors they called from the sky.