Never Bet Your Life — George Harmon Coxe

His job was to protect a friend from suicide, but he didn't count on murder

After a car crash takes his daughter's life, John Gannon doesn't want to live anymore. He tries twice to kill himself -- first by jumping, next with pills -- but doesn't succeed. His doctors recommend a beach vacation with close supervision. For a week Dave Barnum watches his despondent friend drink, fish, and gamble, and gradually grows sick of his ill temper. Finally, convinced that John Gannon has gotten past his suicidal tendencies, Barnum lets his guard down. The mistake proves fatal.

That night, at Club 80, someone drugs Barnum's brandy. By the time he makes it home, Gannon is dead. As he inspects his friend's body, the killer wallops him on the skull and escapes unseen. Barnum's job was to protect John Gannon. He failed, and now it's time for some payback.