A Little Hero by H. Musgrave

Visiting the seaside for a relaxing holiday with his wife, Hereward Musgrave overhears the conversation of three psychiatrists discussing an infamous murder case. The killer, he overhears the celebrated Dr. Byam say, was a madman and almost certainly passed his insanity to his daughter. But what is Mr. Musgrave’s horror when he realizes that the woman they are discussing is his new bride! And, a worse horror yet – Dr. Byam’s diagnosis of her psychotic nature seems to be verified when he is found dead the following day, stabbed through the heart!