Maybe Michael was really jealous that sonny dragged home an orphan waif who was adopted into the family, given a good education and set up in an exceptional law practice beneficial to both himself and the corleone family.
Could very well be. Jealousy is always a factor in family relationships. And, as we saw in the flashback at the end of II, Michael really resented Tom's working with Vito on Michael's future. Michael also may have resented that Tom's inability to anticipate that Carlo would sell out Sonny--dragging Michael into the Donship.
But, I think the answer is in Michael's controlling nature. Tom was Sonny's choice for brother, not his. Tom was Vito's choice for consigliere, not his.
