Worst sportsmanship ever implies that this is a sportsmanship issue in the first place. Biggest dick move ever implies that it was a serious dick move.
Was there a prize? Was there money on it? Was someone really in need of a win, in order to keep his ego from deflating?
Or was it a 3-way game... An automatically-biased system that got further bollixed by random mission rolls, and you got worked up because you didn't win?
I saw someone end a summer campaign by forcing a team-up with the leader, and all but that one person (who should have won) split the prize money, including that almost-winner's partner (who threw the game).
I saw someone smash a well-painted FW greater daemon over a bad due roll.
I saw a ton of other examples. And many of them were caused by people who took the game FAR too seriously and felt like they got robbed. Just like the OP.