That's what you say. However, "real" Christians from not too long ago said they had the authority. Are you saying they were wrong? If so, lucky you don't live back then or your head would be next on the block.
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.