There's a great deal of irony in the way the film portrays Luca Brasi (Lenny Montana). We immediately hear what a tough guy he is, as Michael tells Kay about how Brasi held a gun to a bandleader's head, forcing him to release Johnny Fontane from a contract. He's the Godfather's choice strong-arm, his enforcer. And then… we see him get murdered almost right away.
It's a clever tactic on the part of Puzo and Coppola—our expectations are totally overturned. You're thinking this guy might wind up being an important player during the rest of the movie, but he's whacked straight off the bat.