Simo Hayha had a fairly boring life in Finland. He served his one mandatory year in the military, and then became a farmer. But when the Soviet Union invaded his homeland in 1939, he decided he wanted to help his country.
Since the majority of fighting took place in the forest, he figured the best way to stop the invasion was to grab his trusty rifle, a couple of cans of food and hide in a tree all day shooting Russians. In six feet of snow. And 20-40 degrees below zero.
-snip-
Over the course of 100 days, Hayha killed 542 people with his rifle. He took out another 150 or so with his SMG, sending his credited kill-count up to 705.