If someone threatens they will kill me, is it okay that I threaten to kill them and their whole family?

Mr. Harkess is correct. Why in the world would you think that threatening him back and his entire family would be OK? That is simply nonsensical. Both of you could be charged with menacing and possibly felony menacing, depending on the circumstances. I am concerned that you even thought of that as a response.

It's not only not a legal defense, it's rather dumb. If you actually believe that a person intends to kill you (life threatening situation) it makes no sense to escalate things by making threats to them and a group of third parties. The right response to a credible threat to kill you would be to remove yourself from harms way, contact law enforcement, and if you are actually faced with the use of deadly force by an assailant you might decide to use force against that person to defend yourself. Threatening to harm a bunch of other people is not helpful, could be illegal, and is not very smart.

No. You're confusing it with self defense. There is no legal justification for responding to the crime of threatening to kill someone with the same threat. Did you have the opportunity to walk away to avoid harm? Threat of future injury is not enough to justify making a threat to kill someone. In fact it can be charged as a crime. Serious consequences can follow from responding to a threat with a threat. Contact the police and file a report, let them deal with it instead of verbally duking it out with your assailant.