Shock

From Wikipedia:
Shock is a 1946 film noir directed by Alfred L. Werker.

The film tells the story of a psychiatrist, Dr. Cross, (Vincent Price), who is treating a young woman, Janet Stewart (Anabel Shaw), 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. It also stars Lynn Bari as Dr. Cross's nurse/lover, Elaine Jordan.

As Stewart comes out of her shock, she recognizes Dr. Cross as the killer. He then takes her to his sanitarium and at Elaine's urging, gives Janet an overdose of insulin under the pretence of insulin shock therapy. He can't bring himself to murder her in cold blood, though, and asks Elaine to get the medicine to save her. Elaine refuses, they argue, and he strangles her. Dr. Cross saves Janet's life, but now faces two murder charges.