When Michael leaves on his mission he has two primary objectives. 1) Make a play for Cuba which is the most lucrative, sought after business opportunity for organized crime: 2) Identify the traitor in his organization. Does he want to kill Roth? Yes, but he needs Roth to get the Cuba deal. The movie goes into great detail on how much of a gold mine that Cuba represents. As much as Michael wants revenge, he is a calculating businessman first and will wait for his opportunity to kill Roth. Roth holds a lot of clout, plus has the connections to the Cuban president. If he kills Roth, not only does Michael lose Cuba, he also faces the wrath of the other families.
A key turning point is when Michael sees a rebel blow himself up in sacrifice of the revolution. From what he has seen in Cuba, he concludes that the Cuban government is doomed to fall. He alludes to this at Roth’s birthday party in the company of the other mafia partners. He is signaling to the other families that he is pulling out of the deal. Since Michael no longer needs Roth, as there will be no Cuba deal, he makes the move to kill him and Johnny Ola both (failing at Roth).
So the Cuba venture turns out to be pretty much a failure. No deal. Roth still alive. Michael identifies the traitor but its his own brother.