Mangano must have never seen it coming. He may have argued or yelled at Anastasia, but di don't think it would lead to him being killed. Anastasia killed Mangano, was killed in turn by Gambino and others, then Gambino's successor was killed by Gotti.
The Gambino boss spot doesn't have a pleasant history. Gotti died a slow painful death of cancer doing life in prison, and the last boss was killed by a kook. The quality of assassins has even gone downhill.