Some (most) people simply do not like losing. It's pretty simple, really. Some people think they are the best at video games, albeit nothing else, so if someone beats them at that, what do they have left? They'll claim you cheated or you used dirty tactics; whatever it takes to reassure them that they are still the best. They are delusional, however. You can't be the best without beating the best, and that's common sense.
I don't mind losing especially if you are outskilled. That is one way to get better, to play someone that is better than you at the game. But that being said, I do hate playing someone that has the match one, has beat the devil out of you and still won't go for the win, instead adding insult to injury by dragging out the game to the point of trying to humiliate you. Is it too much to ask or expect that people might have a little sense of sportmanship, class, and fairplay about them? (Sorry but those three things might actually all be the same thing)