None of the Michael motivation really makes sense. Had he been under some spell from the "Others," sure. Some weird mind-meld thing. But without the mental voodoo, none of it works for me. Michael's not a killer. I understand they have his son, but he could have tried one, two or 10 different options instead of killing Ana Lucia (I'll give him Libby as a frightened reaction, a post murder "accident.")
Let's say one of his options was this: He comes back and tells Jack: "They've got Walt. They want Henry back, plus you, Sawyer, Kate and Hurley. If I don't deliver, I'll never see Walt again. We need to make a plan."
Hey, it's a start. And you're not a cold-blooded murderer. Right now that's a storyline that's tough for me. Because Michael can never go back to who he was. (Unless in the end this was one big lab experiment about human behavior and Michael proved to someone in a white coat that, yes, take a man's kid and he'll kill people to keep his kid safe. Experiment over, wake up, none of this really happened). But until that happens, the actual character of Michael is forever changed. He's a pariah. Which, of course, makes him a good bet to be killed off next week. Word to "good" Michael, who ran through the woods in flashback to get Walt: When you see one of the "Others" and you've got a gun - use it. Shoot to kill. You're not living on MySpace Island. Stop being friendly. Kill.