Sometimes I feel like the special ops guy from Eric L. Harry's book "Invasion".
The scene that typifies me goes something like this:
He (the spec ops guy) ambushes a Chinese convoy that's refueling their trucks. He shoots quite a few of them, but does not pull the trigger on a few of them during this attack for some moral qualm or another. Then, since he's scanning left and right from the front to the back of the convoy, he comes across people that he hasn't shot before and shoots them this time around, and he himself doesn't know why he spared them the first time around or why he does shoot them when he sights them again.
This is quite typical of me. For instance, in Oblivion, I could fight hard to save all the guards from one of the big ol' gates that opened on top of a town, but when I'd get caught stealing in that same town, I would have no qualms about cutting them to bits to protect myself. Halo 1 was the same. I'd go out of my way to save as many marines I could at times, but would merely shrug if they did get in the path of one of my masterful grenade throws.
Oh, and on the topic of the vaunted MW2 airport scene, I was reported to have uttered: "Haha! DAKKA DAKKA DAKKA!"
Quick edit: I remember people whining about how you had to kill dogs in various Calls of Duty. (Call of Duties?) But where were those people when you also had to kill other human beings? Hypocrites, ranking canine lives higher than human lives.