I don't understand why you think that since you killed a human being then anything goes afterward. After the state executes a man they don't throw the body around like a doll. If a man shoots someone in clear self-defense he doesn't mutilate the body. The same in war. In war people are expected to kill and die, but they are not expected to do a lot of things like poison the water supply of a town, target a working hospital, slay POWs, or abuse the bodies of the dead. Besides the moral principles involved, there are very serious pragmatic ones as well: These types of incidents make our military missions 10 times more difficult and put more people in more risk.
There is a very real amoral streak that's popular among younger, libertarian types that's been heightened by our post-modern legacy of cynicism, desensitization, and irony. Principles of virtue that this country was founded on and tried to cultivate are slipping as honor, integrity, and respect lose value. We will not last long without them.
This is a reprehensible act.