In India in the eighteenth century, British soldiers with the skill to shoot an elusive bird, the snipe, were called "snipers." The word morphed to describe those able to kill each other, notably the enemy in war, one at a time.
The sniper - or marksman, or sharpshooter, as he was also called -- fought on both sides of the American Revolutionary War and remains a military mainstay today. The First World War was the war between the trenches, a gunshot apart. The addition of optical sights to the guns in the early twentieth century gave the sniper a new relevance, such that in due course each side used snipers to kill the other side's snipers.