The way I see this is that most games (especially shooters) can be obsessed over by those with low maturity.
This, paired with the anonymity things such as xbl have make people do things they would never do in real life. Another part of this is that if someone develops their own standards of how things should be done in the virtual game, which they really can not control beyond arguing and threatening (this is sometimes accompanied by a lack of ability to control their real life, such as being abused or having parent control freaks), they may start to use bullying or lying (about what they can do, like hacking) to keep that semblance of control. If consistant, this can cause an elitism/dominance complex in the person, which is why they may be so aggressive in asserting their empty dominance over others. All (or most) of these things combined, and what you described is what you get.