Runescape? Should I stop my 11yr old playing it??

When he's been playing it, he becomes very disruptive and abusive, even violent when asked to come off the game and starts freaking out saying that he has to complete tasks before his character gets killed or someone takes his character's identity. It's like he is obsessed, almost begging to be allowed... show more When he's been playing it, he becomes very disruptive and abusive, even violent when asked to come off the game and starts freaking out saying that he has to complete tasks before his character gets killed or someone takes his character's identity. It's like he is obsessed, almost begging to be allowed to play the game. Whilst on it and immediately afterwards his behaviour in general deteriorates rapidly to the point where if you talk to him, purely asking questions etc, his responses and reactions are totally disproportionate to the situation as if he's been severely disciplined just by speaking to him, then he becomes defensive, hurling abuse at any and everyone coupled with completely crazy behaviour bordering on violent.
I don't want to prevent him from using the PC as it has its place for general 'edutainment' - educational games for kids, but this, I'm not sure is appropriate or even healthy for my son.
Update: Thanks for all the answers, lots of useful information there. He used to be on the PC more, we limited him as we thought it was better for him if we did and only allowed him on for a couple of hours a day on average but not everyday. He found a new playmate recently, new boy moved in nxt door. And they play out... show more Thanks for all the answers, lots of useful information there. He used to be on the PC more, we limited him as we thought it was better for him if we did and only allowed him on for a couple of hours a day on average but not everyday. He found a new playmate recently, new boy moved in nxt door. And they play out together most days, but it is specificaly with regard to when he plays that game that his behaviour is noticably disruptive etc. So far, I haven't voiced an opinion to him either way what I think of the game, I jst noticed his change of behaviour when he played it and wanted to ask others opinion before I decide what's best for him.
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