Mörderspiel (1961)

Klaus murders women. After killing another girl, Klaus is spotted by his friend Kersten as he leaves his latest victim's house. Kersten gives him a ride and Klaus begins to plot the man's death. He's a witness, after all. Nobody else can say Klaus was there except Kersten. They decide to head to a party. Klaus' unhappy wife is there. So are a bunch of other people. Killing Kersten will be difficult. Then they all play a game called... Murder Party!