Christians have no Biblical authority to kill-they never have had-if some said they do, they were liars.
Self defence, defence of country, and defence of the weak from abusers, in my humble opinion is justified to whatever degree that defence must be taken.
The thinking changes with the ages, but remember they would be telling you that you are a heretic for saying so, and these were people who were "ordained by God" for their positions of religious authority. I personally prefer the modern interpretation that you and most Christians have these days.