After a decade of minor film roles, 1974 saw a breakthrough for Merli with his first starring role in a remake of romantic drama Catene, and brought in as lead in the third of a franchise for White Fang to the Rescue, in part due to his resemblance to Franco Nero.

Betti is a detective who metes out apoplectic violence, and in some way the character was an exploitative imitation of American police thrillers like Dirty Harry and a film of Nero.

Similar to Luigi Calabresi, a real life policeman who though morally upright became notorious after being denounced in media for brutality and was assassinated, the character of Betti finds himself a marked man.

Merli appeared in a few international productions and even a comedy, but remained identified with Betti the toughest hero of poliziotteschi.

The athletic looking Merli enjoyed participating in sport and was unaware of having any heart trouble, but he died suddenly in 1989 of myocardial infarction after a tennis match.