I was a combat soldier and have killed a number of people, with rifle, pistol, bayonet and knife. And perhaps many more with grenades.
I never gave a damn about God, or what he thought. When someone is doing his best to kill you, you get in the first shot, or you die.
It's all very well for Christians to pontificate about right and wrong whilst sitting in a comfortable armchair with a cup of coffee, and quite another when someone is coming at you with a fixed bayonet and hate in his eyes.
And I sleep soundly in my bed without any regrets whatsoever.