I'm the type of person who would say "dude you suck, lemme see that, I'm going to kick your ass" to a good friend in the game situation, and proceed to suck as much as he did. Of course we'd all be laughing whilst. I'm very sarcastic/borderline mean a lot of the time, but the intentions are all in good fun, and I can, and enjoy, getting it back. For me it's not about putting people down to make myself feel better, I just like teasing people I like in a fun way. Sometimes I over do it, yes, but most of the time it's fun. (I do have to work on being a bit nicer though
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So, the whole point of telling you about me, is to make sure your friend isn't like that, and you're just overly sensitive. It appears to me though, that this is not the case, and he is actually a negative insecure person. Being the type of person I am, I can easily tell who's more like me, or who's actually being a jerk for real.
Since he does it behind their backs, and it's insulting, not just teasing, I think he's being actually a jerk, and you need to call him on it, or do something about it yourself, like move.