Morello can be killed in one of three ways. The first is where Tommy, Sam and Paulie chase him to the Lost Heaven International Airport. They arrive at the airport but the car suddenly stopped and was forced to pursuit Morello on foot. He killed Morello's men but Morello then boards a plane to escape, luckily Sam was able to fix the car, they were able to inflict enough damage to Morello's plane, causing it to crash and explode.
The second way is they continue the chase all the way to the broken bridge. Morello had noticed just in time, only problem now was the car was hanging on a ledge. Tommy saw this and pushed the car off. The car falls and after a few seconds Tommy, Paulie and Sam notice the damage. They wondered if anyone survived. Just following, an explosion takes place leaving to the conclusion, whoever survived is now dead.
The third and final option is Tommy drove very close enough to allow either Paulie or Sam to shoot their Thompson 1928 (Tommy Gun) inside Morello's limo to kill him.
With the death of Don Morello, his crime family is no more. With his death, the Salieri Family takes control of his rackets and business leaving them unstoppable.