Okay, I'm confused here. I thought that once Michael left, everyone knew he did the shootings. Assuming this was pretty big news, it would have been talked about between all the survivors. Desmond might have been drunk a lot when Michael was still around, but he would at least have seen him, and he must have heard the story about him later. People would have been talking about it for a while, and if Michael was ever brought up, someone would have to fill Desmond in on what happened, if he didn't already know.