underpromote everyday ignore da haters
Sportsmanship

The childish member was me.
First and foremost, I don't apologize for the first statement because I fail to even see it as rude. The first diagram shows the current position at that point and an explanation of what I was doing.
The trash talking I do apologize for, and I can guarantee to chess.com won't happen again. I was having a bad night and was bitter and crabby.
The third thing I did - chase you around with knights I usually have no shame in doing. Its childish as I'm no longer trying to win, but I can only do such a thing in situations where my opponent is being equally childish. But since I provoked you earlier, I don't have a leg to stand on, and I'm sorry. However, I can't guarantee it won't happen again.
he deserved it
i say not resigning is spiteful, counterproductive, childish, unnecessary, and unsportsmanlike
I notice you didn't include anything about an obligation.
The fact is, one players read on whether a position is lost or not may not be the same as another's. This is particularly true of many king and pawn endgames which can be far more complex than they initially appear on the surface. So while you may feel strongly about a player's not resigning in a lost position being unsportsmanlike, among other things, it is subjective depending on a player's skill level and familiarity with the type of position (and there are many examples where a position that appears lost is in fact drawn, so the familiarity issue goes both ways).
None of unnecessary underpromotion, trash talk or letting your clock run down are subjective. It's clear when it has occurred, and it's clear that each is deliberate and done solely to escalate the situation.
I will, however, say this:
Not resigning a lost position in live chess is, in my mind, a little more offside than it is in correspondence. I think that if the original poster wants to practice his endgame, even if he knows a position is lost, that correspondence is probably a more appropriate place to do it because he does not capitalize on his opponent's time there like he does in live chess and his opponent can simply move on to another game. Asking your opponent whether he minds is probably good form too, particularly when you both know the position is lost.