Was this poor etiquette in the endgame?

Dude missed a mate in one in move 39, with 23 seconds on the clock (more than enough time for the task I'd say). Then he also failed to take your rook next turn. These two moves alone make all his whining invalid. It's 100% his own responsibility that he didn't win this game. He had the chance. No need to feel sorry.

Next time, try avoiding to trade the rooks! It happened near the end of the game, when your opponent was down to a few seconds. Even 1000 rating points above this level, this should give you great winnings odds. It would also be a reward to your stubborn defence, as I believe you put up quite some resistance in the endgame, making it not straightforward for your opponent to convert his massive material advantage.

In general - it is always interesting how chess players often try to make their opponents responsible for their own mistakes :D

Thank you very much for all the guidance everyone! It's always great to get so much information from people with far more experience and skill than myself.

You flagged him, and earned the draw. The poor etiquette was on his behalf, and not yours.

You are NEVER required to resign. That being said though there are some times you should probably resign. The guy went berserk though so don't ever resign to a berskerer you try to flag them.

Remember though, you try to flag them, not let your time run out. So if you sat on move 55 for 3 minutes and let your flag fall that would be poor etiquette

Agree with everyone else, especially with this being a 3-0 game.

Time management is part of the game, I have lost a few games because of this. When I see I am running out of time, I quickly gobble up all the pawns to make sure I have a draw at least :)

The really bad losses are when when you are 1 move away from mate when the flag drops! Happened to me twice now that :)