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.
Ntra la porta tua lu sangu � sparsu, E nun me mporta si ce muoru accisu... E s'iddu muoru e vaju mparadisu Si nun ce truovo a ttia, mancu ce trasu.