Five Dolls For An August Moon/5 Bambole Per La Luna D'Agosto (1970)

George Stark is a wealthy industrialist who invites five business friends of his to his remote Mediterranean island for a weekend of relaxation and business when he introduces them to Professor Farrell, a brilliant chemist who gives investment ideas to the group. But against Farrell's wishes, the group goes behind each other's back to obtain information on Farrell's chemistry ideas and soon the guests and residents start turning up dead one by one as Stark and Farrell must rally the group together to determine the identity of the killer (or killers) despite nobody trusting anyone.

IMDb: https://www.imdb.com/title/tt0065553