After Sonny's gaffe at the drugs meeting, Vito called in Luca Brasi told him: "I'm a little worried about this Solozzo fellow." What he was worried about was that Sol would try to kill him to force Sonny to reverse his decision not to accept Sol's deal. That wasn't Sol being irrational, it was business. Where Vito slipped was in the next sentence: "Go to the Tattaglias, tell them you're not too happy with our family..." as if anyone would believe that Luca would abandon his Don--or that Vito would allow Luca to ply his deadly trade for another family.


I think Michael resented Tom's position in the family. Tom was Sonny's choice for a brother, not Michael's Tom was Vito's choice for consigliere, not Michael's. In the flashback scene at the end of II, after Michael reveals that he joined the Marines, Tom says, "Your father and I have talked many times about your future." Michael fixes him with the Dreaded Corleone Stare and says, "You...talked to my father...about my future?" So, when Sonny was killed, Michael was predisposed to blame Sonny's brother and Vito's consigliere for his death.
Tom knew it, too. Right after the Tahoe shooting, Michael says, "Tom, you're my brother." Tom practically breaks down weeping: "I always wanted to be regarded as a brother by you, Mike"--which tells me that he didn't think Michael regarded him as a brother.