I served.
To be honest, the first thought immedeatly after killing someone is euphorea. A feeling of pure joy and a great sence of accomplishment, an adrenalin rush from hell. This is followed shortly by a feeling of relization and guilt. After a while you begin to reason with yourself, telling yourself it had to be done, convinsing yourself that it was for the better. Ultimatly it ends with acceptance, what's done is done, it's over, it is only a memory. Upon reaching this point, the killer can relax and go on living their life with almost a clean slate as to the past. These stages take time, the middle stages can take years, sometimes decades. Many veterans don't live long enough to reach the last stage.
Like all things though, there are always exceptions. There are those select few of us, who just love it. Those who choose to go back for not only second or third, but 4th and 5th tour of duty in Vietnam or Iraq. The Left 4 Dead survivor, Bill, happens to be one of those 2% of combat zone veterans who love it (hence my name, that and I seem to relate to him the most).
The closer you are to the victim also has an effect on it. To push a button that blows up an enemy tank, is simple. To sneak up behind a guard, who you have been watching for several hours; shift around with discomfort, eat, sleep, smoke, relieve himself, look at pictures of his friends and family etc. And then to sneak up behind him and to grab his helmet, putting your hand on his shoulder, and wrenching it back and kneeing them as they fall to snap his neck. Or to stalk up to them and plunge a combat knife deep into thier kidney (it causes so much pain that it sends them into a state of shock, entierly paralizing them from the pain, all but a faint grunt is heard if at all). Or to loop a rope around their neck from behind, turn around and hoist them up onto your back as you would a sack of potatos, as they struggle and thrash for air (if their neck doesn't break first). At this point, they would give you thier own mother, thier beliefs mean nothing to them now. You are not killing an enemy combatant, or a terrorist. You are killing someone who now only wants to live. Is much more difficult to do in terms of convincing yourself that it was a nessessary evil, but it can and must be done. All of these feelings are natural, a combat soldier must remember that. They are not crazy to think "I just killed a man and I loved it. What is wrong with me!?" It is perfectly natural to feel that way. This feeling is mutual among all combat killers, they need each other to talk, discuss, release thier pains to someone who knows.That is accomplished on a long, ride home from the war. Yes, home. Family and friends simply cannot understand. It can feel like stepping on a bug if you use a preditor missle to kill a truck, but at the same time it can be like trying to kill you very own pet bug, if you are forced to look them in the eye, and break them.
I must also confess that until you are in that moment, you never truely know how you will react. The first time I had to... at a personal level, was a sentry on the airfields of Kuwait, I thought I was going to break down and chicken out. But then he saw me and I had no choice. The image of his face will haunt me forever because I had... HAD to watch the life leave his eyes. It took a long time, the seconds felt... feel... like hours, and I had to hold him from making noise.
Try this... Buy a dinner ham shank and try to sink a knife into it, all the way to the hilt, now imagen it is fighting back, fighting for life.
Now if you will exuse me, I an in need of a rather strong drink.
And a ham sandwich.