This bomb is from the great director John Huston, who directed The Maltese Falcon, The Treasure of the Sierra Madre, The African Queen, and other movie classics. Figuring out why he stooped to directing such a mediocre thriller is a more interesting mystery than figuring out who is murdering the phobia patients in the movie.
Paul Michael Glaser is Dr. Peter Ross, a hospital psychiatrist whose father threw him in a pool when he was a child to cure him of his fear of water. Ross devises a similar form of therapy he calls implosion therapy, that involves throwing his patients into a pool of their own phobias - agoraphobia, fear of heights, fear of snakes, and other fears that are not explained to us very well. Dr. Ross is testing his experimental therapy on 5 convicts who were released from prison for the study, but one by one they are murdered.
One of the patients has a great fear of snakes. His name is Bubba. He's an ex-boxer who was convicted of armed robbery and killing his wife's lover. His therapy is being forced to watch a movie of a burmese python eating a rabbit, and then being forced to touch a harmless gopher snake. When it's Bubba's turn to be murdered, he dies by rattlesnake bite. We don't see it happen, only the bite marks on his face and the rattlesnake on the floor next to him.