Abuse is never justified under any circumstances.
As others have said, resigning in hopeless positions is good practice. It depends on your level and time control as to how far behind is "hopeless" -- top level players will probably concede if they go down a bishop without compensation, whereas in low-level rapid games it can be quite sensible to hold out and see if your opponent blunders-back a Queen advantage or accidentally stalemates you.
A "never give up" approach may give you a handful of rating points in rare circumstances, but resigning prudently allows you to go and play another game you have a better chance of winning rather than eking out a tedious endgame.
At OP's level, he should not resign until he has no chance at all. Don't resign if you're down a piece, you can come back.
There's chess etiquette as the players above are talking about. However, I recommend playing on to the end, ESPECIALLY when an opponent is verbally abusing you. The emotional ones are the most likely players to goof it up.
When you reach the point where you know it is lost completely, and the opponent is calm and cool... resign was made for this moment.
During tournaments, playing for stalemate can potentially either be strategically useful of simply spiteful, my reasoning being:
For the defending side, the time it takes to *maybe* achieve a stalemate and earn one point could often be better used by resigning, starting a new game and potentially earning two points with a quick win. This also brings you one game closer to a double-points streak. Most of these "lone-king vs various pieces" stalemates cannot be forced, but only occur if the opponent blunders. If your opponent makes decent moves you will simply lose after wasting time. You're gambling some precious time on the possibility that your opponent makes a mistake so you can salvage one point, but this is taking you further away from being able to start a winning streak, which is worth more points overall.
Meanwhile, for the attacking side, the difference between a victory and a draw is worth even more. This is due to the fact that a draw terminates a double-points streak just the same as a loss. So if you allow a stalemate to break your winning streak this can cost you lots of points.
If the best you can hope for is a stalemate, then your streak (or progress towards a streak starter) is broken no matter what, and the quicker you move onto a new game the quicker you'll get back on track for those double points. So what this means is that when you're playing for stalemate, your opponent normally has more to lose that you have to gain from the situation. Do you care more about improving your own score, or damaging your opponent's score?
Therefore, in most cases, I find the most sportsmanlike choice and also the most winning strategy is to resign a lost position rather than to play on in the hope of a stalemate. The obvious exception would be when you're playing the final game of a tournament so there will be no further streaks in any case, especially if your opponent is close to you in the rankings such that your position could be affected by the difference between a loss and a draw. But during the tournament, as long as there are more games ahead, it's smarter to resign and get quickly into the next game. By playing for stalemate you are basically sacrificing your own potential to win points for the chance to cost your opponent even more points, which could be interpreted as spiteful.
Obviously, a skilled player will be able to avoid most of these stalemates and always pick up that win. Furthermore, all of this is a feature of the double-points streak system in arena tournaments, and would not apply otherwise. But still, in the particular case of lichess arena tournaments, I'd suggest that playing on in the hope of stalemate usually isn't very clever and is pretty close to time-ticking in terms of poor sportsmanship. At least if you time out your clock your opponent is guaranteed a win. So when you know you can't win and there are still multiple games to go, better to resign and win your next game sooner. Overall it's more points for you and more points for your opponent. Everyone happy :)
You can do anything you want, resign or keeping playing until checkmate. If anyone complains you can block them.
Some people resign too early. There is always a chance the opponent will make a mistake and you can win or get a draw.
I'd say only resign if their is going to be mate on the board in the near future or you are just down a massive amount of material (like a rook or 6 pawns) and there is no compensation and no hope left.
Who knows, I've won some games in lost positions just by having my opponent screw it all up and let me back in the game.
Great point abr, keep it up