Whilst you have a point about the arena scenario, I don’t see how scenario 1 happening was in anyway caused by the dm not understanding the expectations of the party.

From what I understood, it was literally that a player got hit by a spell, fell unconscious due to damage and then decided that was enough and decided to just proclaim her character was dead and leave. There isn’t much to discuss about expectations, that’s how the combat system works. The entire process of rolling dice to see the outcome becomes pointless if there is no chance of failure (the same way that there is no point of rolling for something that is impossible anyway), so you can’t exactly meet the expectation of the player not wanting to get downed even once.

As for scenario 2, it depends on how it was handled. I support the decision to have consequences for one’s actions. Lobbing a magical bolt of fire into a house hold should have consequences, especially if it was done just for the hell of it (not sure if it is that way, but the pyromaniac part of the description gave me that feeling).

But how was the arrest handled? If it was just “you are now in jail” and “you have been knocked out”, then I can see how it might annoy people, but if there was a way to get out (combat/talking/running), then again I don’t think that’s on the dm.

Talking with the players is definitely still a must. They need to understand that actions have consequences, and the dm needs to understand why the players quit.

If it is as dumb as “If anything negative happens to my character, I’m leaving”, then it can’t be helped and the game might be better off without them.

But maybe there is something else that already upset the players, and that was just the breaking point for them. Maybe they felt the encounters were too tough? Getting downed instantly might not be all that fun (although they had already spent all their spell slots, so that didn’t seem to be the issue). Maybe something else didn’t sit right with them during the session (a social interaction perhaps).

I might be missing something though, so how would you approach the topic of expectations regarding combat? The only options I see are rolling openly and discussing difficulty, but again, maybe I’m missing something.

The second scenario though is definitely something that can be talked more about when regarding expectations. (Though randomly throwing a firebolt through somebody’s window because of the pc’s pyromania does lean towards “that’s what my character would do” territory, if it was completely unjustified/random)