Yes, resentment on top of which, as Michael perceived him, his ineptitude. In that regard, I keep forgetting that Vito's near murder took place on Tom's watch. That only added fuel to the fire of Michael's resentment.
Tom was not responsible for Vito's safety, Clemenza was. He hired Paulie.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
