The Beast Must Die

Everything About Fiction You Never Wanted to Know.
Have you guessed who the werewolf is? You have 30 seconds to give your answer.
This film is a detective story-- in which you are the detective. The question is not "Who is the murderer?"-- But "Who is the werewolf?" After all the clues have been shown-- You will get a chance to give your answer. Watch for The Werewolf Break.
—The opening narration

And that's essentially the gist of it. Rich hunter Tom Newcliffe invites several people to his mansion, the reason being that he desires to hunt for a new, more dangerous game: a werewolf. And he knows that one of his guests is one.

Starring Calvin Lockhart, the film was released in the 1974 to cash in the Blaxploitation craze. It is based on the novel There Shall Be No Darkness by James Blish.

Tropes used in The Beast Must Die include:

Paul: (Looks out of the window) "A full moon! Shame that nobody thought to order-"
Tom: (Enters the room) "Wolfbane?"

Tom: "Tonight is my last chance, the last night of the full moon. And I warn you both; tonight, the beast must die, and will."