In what sense is my last duchess an exploration of abnormal psychology and human evil

"My Last Duchess" is an exploration of abnormal psychology and human evil in sense, when Browning lets us see the evil behind the mask by lettering the speaker condemn himself. He is shown to be a warped individual due to the Duke's callous declaration. As the story unfolds, we can almost not believe our ears and if the author wrote the story in third person, it wouldn't have been so effective.