In I and II Michael uses the same techniques to get people he suspects of treachery to confess before he does them in. Unlike Sonny, who had Paulie whacked because he suspected (and probably rightly so) without getting him to admit it, Michael
always got a confession. He did it with Roth and Fredo, in II and in I with Carlo and indirectly Tessio. I think Michael suspected Tessio would be the traitor all along because when Tessio gives himself away by telling Michael he can arrange a meeting on his turf, Tom says he always thought the traitor would be Clememza, but Michael says no, it was the smart move and Tessio was always smarter. Still he did not act on this suspicion until the whole thing played itself out. At the end, even Tessio admitted it to Tom when he asks if he can get him off the hook for old time's sake. I doubt Sonny would have that kind of patience.