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.
Of course when the Russians heard that dozens of their men were going down and that it was all one dude with a rifle, they got fucking scared. He became known as "The White Death" because of his white camouflage outfit, and they actually mounted whole missions just to kill that one guy.
They started by sending out a task force to find Hayha and take him out. He killed them all.
Then they tried getting together a team of counter-snipers (which are basically snipers that kill snipers) and sent them in to eliminate Hayha. He killed all of them, too.
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.
Since everyone they had was either too dead or too scared to go anywhere near him, the Russians just carpet-bombed everywhere they thought he might be. Supposedly, they had the location right, and he actually got hit by a cloud of shrapnel that tore his coat up, but didn't actually hurt him, because he's the fucking White Death, damn it.
Finally on March 6th, 1940, some lucky bastard shot Hayha in the head with an exploding bullet. When some other soldiers found him and brought him back to base, he "had half his head missing." The White Death had finally been stopped...