Michael could not have known at that point that he would kill S&M. But it's a clear statement to Kay that, at least for the time being, he "belonged" to his family, and not to her--that his relationship with her would have to take at least a temporary backseat to whatever he had to do to help his father. So, in a way, you could argue that it marked the beginning of his move to Donship.
Thanks for the info everyone. I think what Turnbull said clears everything up.
I think you are correct TB, and I would add that Michael sensed that Sonny was not up to being a good Don. In the brief time after the hit, Sonny is out of control and not thinking clearly. Luca has turned up dead, and Sonny has ordered Paulie's execution in front of more people than he had to. I think Mike
was unsure how he would fit in, but his telling Kay that he now belonged to the family, and the brilliant instinct he showed by going to the hospital only to find his father unguarded sealed it for him. I think even if there had been gaurds around, Michael was going to the hospital to tell his father "I'm with you now, Pop."
