as far as desmond really being dead, we dont really know at this point. we will have to wait for next episode and see, personally i dont think he is.
and for the part about desmond hitting locke, i have a similar theory. libby was having memories from the island about hurley, and hurley also eventually remembered. i think that desmond had similar memories from the island, specifically the memory of locke pushing him down a well. this is why i think desmond hit locke.