My living among murderers would not justify me in committing murder, and on the other hand it would be no reason why I should not fight in the defence of my family, if the arm of the law were unable to protect them.

That other nations carry on unjust wars is no reason why we should do likewise, nor is it of itself any reason why we should neglect the means of self-defence.
It may seem, to us short-sighted mortals, better that we were placed in a world where there were no wars, or murders, or thefts; but God has seen fit to order it otherwise.

Our duties and our relations to our fellow-men are made to suit the world as it is, and not such a world as we would make for ourselves.
We live among thieves: we must therefore resort to force to protect our property--that is, to locks, and bars, and bolts; we build walls thick and high between the robber and our merchandise.

And more: we enact laws for his punishment, and employ civil officers to forcibly seize the guilty and inflict that degree of punishment necessary for the prevention of other thefts and robberies.
We live among murderers: if neither the law nor the ordinary physical protections suffice for the defence of our own lives and the lives of our innocent friends, we forcibly resist the murderer, even to his death, if need be.

Moreover, to deter others from like crimes, we inflict the punishment of death upon him who has already taken life.
These relations of individuals and of society are laid down by all ethical writers as in accordance with the strictest rules of Christian morality.