Same weak argument as:
Animals kill each other in nature
Killing is natural,
Therefore it is ok for humans to kill each other
Great logic bro...
animals don't kill eachother for the sake of killing.. It's for survival. Be it to save there own hide or for food. In the event of one human put against another, that is survival, and you have a right to survive. Hence why if the attacker gets killed by the attackee, it's less common to see them get charged with murder since it was not pre-meditated.