Ask a soldier in war how they can shoot to kill.
Because the guys you are killing are trying to kill you, they're a threat to your country which holds everything you love, and because they've pretty much already sealed their fate just by agreeing to be in the army (it's very likely they'll die anyways even if you DON'T kill them). Killing someone who is deadly, already very well equipped to hurt you and who you love, is perfectly willing to do it, and has already agreed to lay down their lives for their country is a HELL of a lot different from TORTURING an innocent animal who can't defend itself and has done absolutely nothing to wrong you until its dead.
It's simple, you just switch off, and pretend it's nothing more than an animated object. My father and one of my teachers were both officers in the army a while back, and they both said the same thing. You won't fire a single round until you learn to shut it all away. There are one or two that enjoy the killing, but they aren't actually very good at what they do, and don't last very long.
Or just consider the fact that they're trying to kill you and destroy what you love.