What's wrong with me?

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I'd consider myself a fairly average adolescent. I'm sort of fit, I enjoy school, have a decent social life, love to read e.t.c. But over time I have begun to notice that there is something wrong with me. On a consistent basis, whenever I meet new people or whenever I am around people I already know for too long, I start to imagine myself performing horrific murderous acts on these people. I often have to excuse myself from the room to keep it in check. I have never felt these urges towards my family or my closest friends however. I am far too afraid to go to a real doctor or admit this to a shrink, nor do I trust Internet M.D. at all really. Is there anyone out there that can tell me what's wrong?