I've come close on a couple occasions.
The first time was in Kosovo, when I had a round in the chamber, safety off, sights on a Serb soldier when he decided to scoot back towards the border. Chances are, if my company commander hadn't prematurely launched a flare, I might well have had to shoot that soldier. I'm kind of happy the CO launched it too soon.
The second time, I actually fired some 81mm mortar Illumination rounds above the heads of two Afghani men at Kandahar Airfield, in March '02. I was the Assistant BDE Fire Support NCO during OP Anaconda. If those guys had decided to act squirrely and fire at the Apache I had overwatching them, or at the Canadians observing them, I was fully prepared to fire them up with mortars, the Apache, or with Canadian Infantry dismounts. Luckily, they decided that it was better to run away and fight another day. (If you consider sneaking around at night, emplacing Anti-Tank mines on previously cleared roads fighting.)
Other than those two times, I've never had to face down an enemy or make the decision to end another humans life. I do know that when I was in those situations, I had the full power to kill them, but in neither situation would it have been right. Granted, none of them were likely candidates for sainthood, but they hadn't done anything that warranted me killing them. I believe that's what seperates us from most of our enemies. We know when to kill someone and when it's not necessary, and we act accordingly. Many of our enemies kill indiscriminately, with little or no regard for whether it is morally right or wrong. And THAT is one of the things that makes America great.