You have to neutralize any of the things he can threaten you with. Either blow your cover "first" or call his bluff by ignoring him (feeling that if he blows your cover you will be okay). I would ignore him and/or go on the offensive and point out to him that his negative, excessively hurtful acts aren't exactly making you want to stay with him but showing him up for the cad/jerk/emotionally abusive person he is and think of some way to support yourself if he does blow your cover/hurt you or your loved ones.

If you don't feel you can tell your husband and the guy does blow your cover, then immediately apologize to your husband that he had to learn about it in such a hurtful way and that that is why you broke off relationships with the guy; you found/discovered he was exactly the way your husband will feel and you realize your husband is the more loving, stable, (add a zillion good/heavy adjectives :-) man for you. Imply that you broke off "long" before and tell your husband he's been harrassing and threatening you as a result and his words and actions are not to be counted on. Just make sure you thoroughly discredit the other guy's version and actions about things. Don't lie though :-) Just tell your story of what you "see" in your husband versus in the other guy and apologize well for any hurt you may have done your husband both literally and in getting mixed up with this guy who then has hurt your husband with what he has to say.

Someone who truly loves you would not hurt you or yours/would not "retaliate" no matter what you did/said! Your husband may/may not be able to forgive you and may/may not want to work things through. Good luck with whatever happens and I'm very glad you know this guy is not good for you and you don't want to go with him for sure now!