I think you're both wrong, so there! Hmm...what was that about childishness?
It's not easy to get away with murder in the UK today, not even if you murder a random stranger (i.e. there's no clear motive linking you to them).
Those shocking murder rates in places where law has broken down are still very low, usually occur during what is effectively a civil war and the proportion of people who commit murder there is much lower than the murder rates because many of the murderers kill more than one person. Most people are very unwilling to kill. As h4rm0ny said, this tendency towards not killing is so extreme that it's a serious problem for the military and they have spent a lot of time finding ways to train people to be willing to kill.
I can give you the name of the study h4rm0ny was referring to - it's called "Men Against Fire" and was done by Brigadier General Samuel Lyman Atwood Marshall. The two most pertinent findings was that only 25% of soldiers in a position to fire on the enemy fired in the direction of the enemy and only 2% shot to kill, i.e. actually aimed at an enemy soldier. The figures are argued about, of course, but they're plausible. There are some more direct pieces of supporting evidence, e.g. in WW2 in the USAF 1% of fighter pilots accounted for 50% of enemy aircraft shot down. Presumably some of that was due to the very best pilots being much more skilled, but that's a huge difference. Nor is it a new thing - there are recorded tests going back at least 300 years. Unsurprisingly, military leaders wanted to know how effective large scale use of guns (still a new thing ~1700) was and so they conducted tests. Accuracy in battles was less than 1% of accuracy in tests, with almost all shots going over the enemies heads. The situations were firing on a formation of men, so the only way to miss them all was firing over their heads (firing low wasn't possible with muskets in a formation of rows of people). A lower rate of accuracy would have course be expected, but not that much lower and not all over their heads. Shooting to miss was clearly a very common thing.
Murder very clearly is not "how we acted for a long time". Sure, humans are on average meaner and more violent than most other animals. But not by
that much.
Lindybeige has an interesting video talking about shooting to kill and human nature: