I think Colombo was shrewd enough to calculate the odds on this one. The Bonanno/Magliocco plot was an egotistical attempt to install Joe Bananas as head of the Commission.Knowing that Magliocco was not respected by the other bosses,and that in addition to Gambino and Lucchese,Maggadino was a target,Colombo made the right call. He knew that there was almost no chance for the plot to succeed. By tipping off the potential victims,he assured himself a nice future. What I think is the most interesting detail is that Colombo wasn't whacked after the plot was destroyed. Given the mob mentality,the feeling is usually "Even though he did us a favor and saved our lives,he still ratted out his own boss,so how can he be trusted?"