I'm going to throw myself in with those thinking there are multiple killers - we saw one gunshot on Nick when he went down. It was one blotch, and it was relatively clean. When he came through the square a few minutes later, there were multiple shots on his shirt. Someone shot him. Then someone *else* came through and shot him multiple times to be sure it "stuck". Now, the question for me is not so much who shot him, but who wanted to be sure there was a kill shot that he couldn't survive (in which case, I would shoot him through the head, but this *is* Salem and we have the fabulous Doctor Dan who can save anyone except, well, someone with significant multiple gunshot wounds).