If you were to stipulate that the rabbit is the only source of nourishment available to the fox, this still in no way justifies murder. The fox would have a moral obligation to starve to death.

How different is it when soldiers are at war? They must kill or be killed. If the fact that enemy soldiers will kill them if they don't kill the enemy first isn't enough justification, what is?

Should the soldiers on each side sit down and argue out the moral justification for the war first, and the side that is unjustified should surrender?

But somehow it seems like they hardly ever do that....