When he and Connie got married - Vito was specific not to let Carlo into "the family business". He was "to be given a living" but nothing more than that.

Yet Michael wants to make Carlo his "right hand man" in Vegas, and then ends up having Carlo killed after the heads of the five families were killed.

I guess i am confused. Did Vito see something in Carlo that Michael didn't see, until the end, or was Michael using Carlo to smoke out someone else?