It's like this is the most popular game with stream monsters, seriously. But, it's also just people. If there was a one-on-one first person shooter, plenty of d-bags would send hate mail to someone who LOSES, but they don't tend to because when you're fighting a team it's hard to single out any one person on the team as having been the biggest loser.
You see this douchebaggery in DOTA or LoL too, during in-game text chat or the post game lobby chat. Based on the chatty bitches I've seen there, they're power tripping nerds who get off on imagining that they're making someone they've already beaten cry. Saw one guy, once, who when a person he was trashing wasn't bothered and just laughed at him, actually said "sure you're not really laughing that means you're angry at sucking so bad". I knew the person who was laughing; they were literally laughing, in real life. They weren't bothered by losing. But the d-bag was convinced his "victims" were all crying and he was drinking dem tears.
Oh man, people don't even know what a bad community is until they've played DotA. I was really into that game for a few years, and I still think it's one of the best games ever made. I tried to get my wife into it, and it just wasn't her thing. Her only match, though, was filled with tons of people raging at her, telling her to "get the fuck out of the game", etc. Yeah, she went 0-13, but she was still learning the buttons!
She thought it was absolutely hilarious, just like your friend did. After a while, she started typing "lolz" every time she died. Of course, my wife is also one of those people that would sit in Barrens chat in WoW just to talk about Chuck Norris - she is an odd and lovely bird.
My first DotA match was abysmal. All 9 of the other players were pissing on me for any mistakes I made, and I didn't even understand what the point of the game was yet. I fed hard, no doubt, but no one tried to teach me anything. Luckily for myself, I'm an ardent self-educator when it comes to gaming, so I learned a lot and did well. But even as someone who had gone Beyond Godlike with 80% of the characters in the game, I still had bad matches, and the rage and spittle was always there.
Oh, and if you happened to be grouped with one of the self-righteous assholes that thinks he's just the best DotA player ever, god help you if you happen to "steal" one of his kills. He'll grief you for the rest of the match for "losing the game" for the team. Or he'll intentionally not help you if you're being ganked, or will just sit around and KS you for the rest of the game.
Ugh.
Don't give him ideas or else we'll get another lame Dollar Tree joke.
*shots fired*
That joke had the Dennis Miller Ratio going for it.