Often times, BM can cause the person on the receiving end of it to react to it. In my personal experience being sort of a shy, introverted and self-conscious person, I take offense to it and my mood drops, sometimes even causing me to put the game down. Others might be more positive to it, acting back with bad manner and forcing an argument, which is just fuel for the fire.
Obviously communities like the SC community and especially the LoL community have a proportion of people who have no fear or conscience about what they say to people.
From what I have seen, those who BM are trying to accomplish one the following:
1. If the BMer wins, they trash talk their opponent by telling them how bad they are and that they are scum of the earth and should stop playing.
2. If the BMer loses, they can either blame the game for their loss, tell their opponent that they are bad still and that they shouldn't have won, or just try to make their opponent mad in other ways (ex. floating buildings in SC, etc.)
3. Before the game even starts, the BMer can cause chaos in a lobby by inciting hatred on others. This is especially prevalent in games with voice chat or tangible lobby chat rooms.
Taking a step back from the game and looking at it from an outsider's perspective, when you boil everything down, games are supposed to be fun and something that uplifts ones mood (sans horror/depressing games). In the SC community, most of us say GLHF before our matches, signifying that it is still a game, and even though one of the players will win and the other won't, it is still fun and exciting.
But, from personal experience, BM has at times made me personally upset so much that I can't even think about anything else. Obviously I personally shouldn't let it get to me so much because I will never see that person IRL or the likewise, but it is still an adverse affect on my mood.
So I want to ask a question: Why do people do it in the first place?
Is it because of them being jealous of their opponents skill? Is it a direct correlation to their true personality, being an angry bully towards those they feel are superior? Do they personally enjoy gaining entertainment from other people's expense? Are they striving to achieve a false sense of superiority? Do they feel security knowing that they are mostly anonymous?
My opinion: It's a bit of everything. Of course no one likes to lose at a game. You spend your time trying to win only to not. You get mad because you run through all the steps in your head as to how you could have changed the result. In the case of you winning and BMing your opponent, you are looking to spark a reaction from them. Just like a typical troll.
Bottom line question is, why are there so many people who do this, and will it ever reach an end? Because as it looks right now, even reporting someone for this in game won't stop it even though it is meant to.
EDIT: I also wanted to inquire about if you have BM'd and why? Are you the type of person to react to those BMing you?