They're born into mafia families, their memories are long, they want to avenge deaths from decades ago, etc.
If it was Marco Di Lauro who ordered this Gaetano Marino to be killed, then he is going against any revenge logic. He had lost no close relatives in the war between the Di Lauros and the Marino-Amato group while Gaetano Marino has lost a father if memory serves me right, so theoretically it was him who should have taken revenge on Marco Di Lauro. Maybe they killed him just to prevent that.
