How could Michael have known for sure that, facing the chair, Rocco wouldn’t have betrayed him? And if he did, the police would know that Neri and Tom were witnesses when Michael gave the order.
You're right...he couldn't know for sure. I think it definitely was a case of Michael slipping. He actually followed Sonny's behavior.
If he had been more cool about what his true intentions were about Roth, I don't think he would have incriminated himself. But he was so focused on belittling Tom, that he threw caution to the wind. Great analysis.
Michael does this before, when he tells Fredo his intentions with Roth when they went out for their brotherly drink. Again, Mike is so focused on traitor, he doesn't see he's right in front of him.
