It may not seem like it matters online, in a game, but you are training yourself in very bad habits by acting as though there are no consequences to showing no manners online. Just because you are not looking the other players in the face, and cannot hear their voices, does not mean that they are any less real, that they are not hurt by your actions, nor does it mean that they exist outside of the real world. What you are doing is deluding yourself into thinking that you can keep the two separate and distinct. You can't.
It starts by acting as though the faceless entities online are unimportant and not worthy of consideration, and that it doesn't matter if you are rude to them. It progresses to being rude to other people that you can't see. Like the customer service representative that you have to call on the phone because your internet isn't working. Oh, you justify it because she's incompetent and slow, so you snap at her and demand to speak to someone else, or because he doesn't speak English very well. So it's not really wrong of you to argue with them, it's not insulting them if it's the truth, right? Or how about that kid working the drive-through at the local fast food joint. He's obviously mentally challenged or deaf or both, that's the only reason you had to repeat your order three times to him. So it's not wrong that you yelled it at him the last time, right?
When you start dehumanizing the other people you interact with, you make it that much easier to continue to do so. And you can say "oh I wouldn't do that" all you want, but it's a fact of human nature. Human beings are capable of horrible atrocities against each other, just look at the history of war crimes dating back for millennia. It all starts when people stop looking at other people as if they matter.
