The ego, something that's a part of the human nature.
Gonna rage over it and be a hypocrite about raging?
And on a side note, one person's definition of fun is another person definition of hell.
In other terms, you're point of view in fun may just be destroying another person's fun, which then leads to rage.
Since no one ever negotiates over such trivial matters over the internet , this rage will continue to exist in Gunz as well as all other competitive games.
And another secret. There's these things called trolls that lurk around in games for years now. I don't know their purposes, but there's some kind of relation with rage and them.