Tessio would never betray Don Vito Corleone, as he knew Vito from his early days and knew he was a man to be reckoned with. The smart move would be to stay with Vito, but when Michael came into power as the Don, Tessio did not think Michael could handle being the Don. He felt it was the smart move to betray the family. Interesting thought of how he could have betrayed the Corleone family early by pulling off his men, but I do not think this was the case as he could have then be more suspected of betrayal if the police did not arrive and arrest his men. Tessio was in charge of protecting the Don and no other matetr was more important than that.