There are two things which cause toxicity in the gaming world (or basically any place with competition) in my experience:
1. The first one is what you experienced at the "gaming-con" or whatever, in which a person derives their self-value from their own abilities at given game. While this may sound silly (basing your own self worth on a video game?), it's not sillier I think, than basing your self value on your job, your ability to attract women, how nice your car is, or anything else for that matter. These people have simply chosen to base their self value on their "skills" in a particular game. So obviously, when they're winning, they feel like they're on top of the world and like nobody else can beat them. However, once they begin losing it begins to tear at them, grate them, make them feel extremely uncomfortable, frustrated, and ultimately worthless, which is why they'll often take their anger out on an inanimate object or, in some cases, another person.
2. The second element which causes toxicity in the gaming world, and especially in the MOBA world, are the player's expectations. Players aren't simply playing to have fun (though that's certainly an element of it), they're usually playing to win. Often, they *expect* to win. They also expect their teammates to have a certain level of skill which matches their own (this is obviously completely perception-based, and difficult to quantify). You'll notice that when things begin to go against the grain of a player or players expectations, that's when all the yelling, screaming, and shitstorming begins. Let's say a player makes a really big mistake, or does something contrary to what he was *expected* to do. Suddenly he'll be at the behest of verbal insults, chiding, and the extreme frustration of at least one or members of his team. Sometimes people will even say, "Why did you do that? I expected you to do this." Basically, in each player's mind, they have a certain expectation for how each player should behave, play, and act. For example, the carry should be farming, the support should be warding, the jungler should be ganking, etc. When people begin deviating from these expectations, that's when the anger generally starts. Some people assume that losing is what causes people to become angry, and maybe to a certain extent it's true. However, I've lost many games in a MOBA where everyone on the team was playing fairly well, the other team was just better or more coordinated. More often, the anger and insult-throwing begins when a player or players actions are completely inexplicable, ridiculous, or unpredictable, and completely deviant from what a member or members of the team expect him to be doing. The MOBA genre is unique in that there are specific expectations (or roles) that each player is knowingly or unknowingly given. Often players will misunderstand or not be very good at these roles, and thus it creates a lot of anger and frustration for everybody else. The key (in my experience) to avoiding this anger is to enter the game without expecting anything at all, and then simply adapting to your team's fluid playstyle instead of raging for not adhering to your rigid structure.
