The Kiss Before the Mirror (1933)

When a lawyer defending his best friend for murdering his unfaithful wife discovers how the accused discovered his wife was in love with another man, the lawyer begins to see the same patterns in his own wife's behavior, and suspects that she too is being unfaithful. Promising his friend that a defense of momentary insanity will acquit him, he also promises to kill his own wife if the defense actually works.