Shock (1946)

Dr. Cross, a psychiatrist, is treating a young woman, Janet Stewart, who is in a coma-state, brought on when she heard loud arguing, went to her window and saw a man strike his wife with a candlestick and kill her. As she comes out of her shock, she recognizes Dr. Cross as the killer. He takes her to his sanitarium and urged by his nurse/lover, Elaine Jordan, gives Janet an overdose of insulin. But he can't bring himself to murder her in cold blood and asks Elaine to get the medicine to save her. She refuses, they argue, and he strangles her. He saves Janet's life, but now faces two murder charges.