Nauru has less than 10,000 people--so their rate may reflect one person being murdered.
During the last century we had both WWI and WWII take place primarily in Europe. Murder was not confined to the battlefield in either war. The primary instruments of death were firearms ~ and once you unleashed the sort of firepower they had at that time you would necessarily end up with many deaths by disease, but also this time by starvation and just bric a brac falling off the sides of buildings. But the primary instruments were firearms and to them must be attributed that enormous number of dead bodies!