Violent dreams of any sort are usually a means for your subconcious to deal with what's happening in your life, especially if you're one to avoid conflict (thus having no outlet). The fact that they involve your best friend probably just has to do with the fact that he's around you a lot of the time (I assume) and possibly is present for the conflict, or lack thereof.
That, or you've watched the Number 23 recently, and have developed an obsession.