iv seen this before.. first of all, hes not playing steam, if you can hear the bg sound it is actually unreal tournament 2004. second he is mad because he is getting either owned by another player or can't load the game properly.
I've been told that this is faked, there's another vid of what appears to be the same kid acting out a different kind (and not funny) of mental imbalance here - http://www.flurl.com/uploaded/Neues_Gangsta_Video_61506.html I'm actually kinda relieved.