Was it a stupid conversation to argue against slavery when that was legal? Saying the law is the law as justification for the laws' execution is just as lazy as calling killing in the name of justice "barbaric" without giving the act of killing serious thought. Human societies with their higher level of thought compared with other animals have come to understand the importance and sanctity of life, so we enact societies and laws to protect it and deter the taking of human lives. When someone comes along that openly and blatantly disregards the importance of the lives of others, punishment obviously needs to be levied, lest you risk the laws protecting life to be disregarded by the masses.
But consider what it really means to say that someone forfeits their own right to life if they don't respect the right to life of others. What motivates that thought? It's fear. Fear that someone who doesn't respect life poses a direct threat to my own existence. It's a legitimate fear. But we are so quick to transform that fear into vengeful justice because the feelings of revenge are powerful ones that can overshadow our fear. But they hardly erase the fear, they just let it fester. We don't want to see others that don't respect life as being humans with their own right to life because that's too hard to grapple with. I promise you that every child in the world starts off understanding the importance of all life. No one sees their first murder/death and smiles. Societies train us to understand that killing in the name of justice, for country, in war, and in many other ways is not only acceptable, but necessary, in order to protect yourself and the lives of those you hold close. The only way to fully achieve that is to dehumanize those other people and call them the enemy, even though their motivations in life stem from very similar base human desires. We are one and the same on the most base level. We are all human and wanting of life and love, even if some of us lose our way as society guides us to transform our want of love into a need to hate in order to control our fears. It's too hard and scary to try to see someone that wants you dead as just another human being with the same basic wants in life, that somewhere along the way they became fearful and grew a desire to harm others to respond to that fear. It's scary because then we start to understand them, and understanding someone you vengefully want dead is terrifying.
We want to protect the sancity of life, but can we really do that if we are ready to throw out that desire because of fear? The answer is no.