Bad smell=dead. Toss it. If you can stomach it, cut the egg open and examine the contents. If the egg were ever a baby, you will find the embryo inside. If there is no embryo, the egg was infertile and you did nothing wrong. May make you feel better. Inside the egg will be a smelly hard yellow glob that separates from the shell very easy. The baby will be along side this glob. I cut into all my eggs gone wrong just to see what may have happened and to learn from that. P.S. Wear gloves.