Those who are against you owning and carrying effective weapons, and using them to defend life, liberty, and property, like to pretend the bad guy is your ethical equal. That his death, as a consequence of his attack on you, is some sort of tragedy.
For an anti-gun bigot to say that using a gun in self-defense “costs a life” makes it sound as though you traded a random innocent life for your life. As if, one day for no apparent reason, you feel threatened so you go out, find some little kid who is minding her own business and kill her as she sits at her “unlicensed” lemonade stand so you can live. It’s not like that at all. (That’s more along the lines of how The Blue Line Gang operates.)
Instead, in such a case, someone has chosen to show you they don’t value your life. I would say they also don’t value their own life as much as they value their desire for attacking you or taking your stuff. They are trading their life for their desire to violate you. They decided on the “game”, they know the rules, so the outcome is on them when it doesn’t go how they’d like.
It’s not that they “lose their right to self-defense” once they attack you. Rights can’t be “lost”. But their right to self-defense doesn’t do away with your right to self-defense, your right to not be molested, and your right to not have your property rights damaged– at their hands. In the current circumstance, chosen by the archator, you have the biggest stack of rights at stake. I hope when the smoke clears the archator is on the losing end. Every time. If fairness were a feature of reality, that’s how it would be. Since it isn’t, you need to do all you can to stack the deck in your favor.
The bad guys, including the anti-liberty bigots, aren’t going to cut you any slack.