People have glorified wars today too; that does not in any way change the fact that many soldiers, despite years of training, cannot bring themselves to kill another human being in cold blood.
Now hot blood; hot blood is different. One of the most common causes of homicide are Crimes of Passion, the reason for that being that when one is sufficiently emotionally unstable or incensed, one's higher brain functions are either bypassed or overridden. At that point the really
old part of our brains, the part that used to be a fish, takes over, and unlike the monkey parts of our brains, the fish part is
not a social creature. It is incredibly primitive and has little more than basic reflex action and fight or flight decision making, which is why after most crimes of passion the person responsible usually ends up in shock going "How could i have done this?"
People desensitize from killing extremely quickly if it is forced upon them, and in the heat of an old battle, with screaming men charging towards you waving various sharp impliments and threatening your life, there's no room for rational thought. The fish brain kicks in. Of course if it
doesn't kick in, you usually end up dead.
Afterwards of course, sometimes the monkey brain can't reconcile what it did, and then you get shellshock, or one of many similar conditions.
It's also worth noting that once a person
is indoctrinated to killing, they usually
stay that way. That's one of the reasons why soldiers often have difficulty reintigrating into society; and also why they're usually kept seperate from it.
This isn't to say that people are incapable of killing other people, merely that it is not
easy to do so and that the vast majority of people are not inclined to do so. If pushed to the limit, anyone can and will kill, but killing in cold blood (which is what the military wants) is much,
much harder.
By the way, you may want to read
this article.