They fight for us and to defend us, but they don't have to enjoy doing it... IMHO.
They do if you want them to be good at it.
Enjoy it, Mani? I don't think so; soldiers do not have to be psychopaths, and most, thank God, are not. What soldiers feel as a result of the killing they have to do varies widely; some deeply regret it, sometimes even to the point of carrying terrible guilt for it the rest of their lives; some take a certain grim satisfaction in killing the enemy while playing by the rules, and a tiny handful actually seem to like it, but most men fall somewhere in the first two categories.
Bones, you asked "who drew the line?". It really doesn't matter; the line HAS to be drawn. It is true, that an infantryman has to balance overcoming his inhibitions against killing (which most Americans have), against having the discipline and restraint to NOT kill indiscriminately. Yes, sometimes, that is asking a lot, because it IS hard to have empathy and respect for a foreign people, when a soldier can't readily distinguish the enemy from the noncombatant civilians he hides and fights among. We learned that the hard way in Vietnam. Nonetheless, that line is clearly drawn by regulations, the UCMJ, and rules of engagement, and every soldier knows where it is. There is no excuse for those who choose to cross that line, and I refuse to in any way excuse or defend such misconduct. It might surprise some people to know that even in Vietnam, where training had not yet completely caught up to the realities of a war where the distinction between combatant and non-combatant was blurred, there were comparatively few incidents of deliberate, willful murder of civilian non-combatants, although the incidents that did occur got a lot more publicity than the far more numerous acts of kindness and compassion by American soldiers toward Vietnamese civilians.
In talking to veterans of more recent conflicts, it's clear that as a result of lessons learned in Vietnam, our military has placed more emphasis on discipline, restraint, and following the laws of war and rules of engagement. This is a good thing; atrocities only help the enemy, endanger one's fellow soldiers, and in addition, are completely unnecessary, and a disgrace to the individual, his unit, and his uniform.
Contrary to myth in some circles, the U.S. military has as good a record of honorable behavior as any on this planet, and a lot better than most. Even those killings of civilians which are truly accidents, and have no disciplinary consequences, are deeply regretted by our troops; no reasonable man wants something like that on his conscience, however inadvertent it may have been.
The vast majority of us, then and now, were not and are not cold-blooded murderers, torturers, or "baby-killers". That is NOT what we were trained to do. The actions of an undisciplined, murderous few notwithstanding, the vast majority of our troops behave honorably and professionally, and have no sympathy for the "filthy few" failures who do otherwise.