Part of the greatness of that last scene in Roth's hotel room is that, by that time, both of them were onto each other--and each knew that the other knew. Michael already knew when and how Roth planned to have him killed (as he told Fredo). And, I'm guessing that, when Michael sent Pentangeli to meet with the Rosatos instead of having him killed (as he told Roth in Miami), Roth had to have been thinking that Michael didn't suspect Frankie in the Tahoe shooting--meaning that he might suspect Roth. Roth's suspicions had to have been fanned when Michael didn't bring the $2M to Cuba, and when Fredo did, he didn't turn it over. I think the final straw was when Roth demanded the money and Michael replied, "I just wanna wait."

But, never underestimate the power of greed to trigger self-deception: Even if Roth had concluded that Michael thought he was behind the Tahoe shooting, he still held out for that $2 million. The clincher: After Michael asked, "Who had Frank Pentangeli killed?" and asked "Who gave the order?" Roth replied with the Moe Green story and emphasized that he "let it go because it had nothing to do with business!" He was telling Michael to let the failed Tahoe shooting go because it had nothing to do with the business they were transacting right then in Havana--desperately trying to buy time to get the $2M and then have him whacked after midnight. And Michael, who could have had Buscetta whack Roth and Ola right there, was desperately buying time down to the last minute to see if he could find out who was the traitor in his family.