YOu mustve missed Bens line where he said 'youd be surprised what any man would do to get his son back'
No, didn't miss that. It just doesn't change the fact that Michael, among other shitty things, murdered two people who did nothing to him in attempt at getting his son back. After Walt's abduction, Michael was a hot-headed loose canon who could not be reasoned with. From his point of view, he did what he needed to do. From everyone else's perspectives, his word may not mean shit. He may very well help Des and Sayid, but I guarantee Sayid won't blindly trust what may just be a wolf in sheep's clothing.