Stanley G. Weinbaum - Dark Other

The Dark Other is a horror novel by Stanley G. Weinbaum. The novel concerns Patricia Lane who is in love with Nicholas Devine, a quiet and gentle writer. Devine undergoes sudden changes becoming cold and calculating. Frightened by this, Lane consults psychologist Dr. Carl Horker who rescues her from Devine. Devine again attacks Horker, and overcomes him. He is then shot by Lane and rushed to a hospital where a surprise is found. (Summary adapted from wikipedia)