Well, to irresponsibly draw an analogy between the Eastern concept of karma and Western Kantian philosophy, an action's consequences in any one case is independent from its ethical implications. Embezzling a penny from a rich corporation is still morally wrong even if they'll never miss the money. On the other hand, telling the truth is the right thing to do even if it may hurt someone's feelings.
A Kantian could argue that executing a murderer who had full mental capacities and acted in cold blood is just, because the murderer knew that killing was wrong but did it anyway, and therefore decided that the moral rule did not apply to him. So how could it be wrong to kill him? The moral rule did not apply to him by his own decision.
And a Buddhist might argue that killing the man would not bring bad karma if the alternative were letting him go free, since karma is gonna off the guy anyway. Why not stop him before he kills again? (Keep in mind that it was the alternative before the gradual establishment of the modern prison system. The most common punishment in medieval times was physical.)
So why would a Neko soldier have scruples about killing an enemy of the Empire? Better to kill him before he kills her friends. The enemy made certain decisions that led up to him participating in a battle on the wrong side, so there's no bad karma there.
Anyway, for some reason, wars were still fought in China and Japan after the introduction of Buddhism. Go figure.
Incidentally, a Kantian would have no problems against letting people volunteer for duels to the death. Those things remained popular long after Enlightenment, and still occasionally occur today. Again, go figure.