WGS' Theories...
I have two ideas for why Michael did what he did...
1. I think the Others forced him to do it to give him Walt back. They say something like, "Hey, we have what you want, and they have what we want. Go in there and get what we want (Henry), and we'll give you what you want (Walt). However, why he shot himself, I'm not really sure with this theory.
Or...
2. Perhaps Michael was thinking that the rest of the group wasn't too hip on the idea of ambushing the Others, so he needed to give the proper motivation. If they see Henry, whom they thought was subdued, killing again, they'll want, nay, need to go after him. So, Michael shoots some people and himself to frame Henry, so everyone goes after him.
Those are my thoughts. Here are some other theories from my family...
1. Henry is in charge of the Others, so that's why all this trouble is being went through to get him.
2. Michael has been brainwashed or something, and is now an Other himself. Him shooting himself will just give him a way to "prove" that Henry was evil, so they'll keep trusting him.
3. (A bit out there) Michael is just going insane from not being able to do anything about Walt. He killed Ana because she was too weak to kill others, and Libby was an accident out of fear. Of course, this still doesn't explain why he shot himself...
Really, most of these are at least semi-plausible, so I'm kinda leaning for any of 'em. I won't be too shocked if any of them turn out to be the truth, really.
Now I just want to see next week's episode.