Nature versus nurture usually refers to the impact of one's genetic load versus environmental factors on the development of personality and behavior. While both are undoubtedly partners in the tango, my vote goes to nurture as the leading man, or woman if she's a dominatrix, in the rotating mirrored ball of existence.
Surely Hitler must have been born with some pretty feisty genes, but had he not been rejected by a Parisian art academy and haughtily dismissed by fellow painters in Paris, WWII might have averted. Hitler did a jig in front of the Eiffel Tower when his troops invaded Paris and settled in for the duration. He could not contain his glee; he now owned the city that had once snubbed him - truly a sore loser, this mad conqueror.
And then there is the story of Ted Kaczynski, the Unibomber, with a last name as phonetically twisted as his mind. He was skipped two grades in elementary school and entered Harvard at age 16; this age differential, which began in childhood, immediately insured his status as a social outcast.
Ted was also unwittingly subjected to an experiment by the CIA on gifted students while attending the almighty Harvard; in the guise of studying young genius, the government tormented these young elite to see how well their stellar brains could withstand pressure, in a shocking case of conspiracy, the university played along. Not a good recipe for awkward Ted, who had been cast as the outsider geek since a young age. It is no mystery that his bomb targets were people in the educational field, as his tolerance for ridicule morphed into murderous payback.
And then the opposite scenario: kids from the belly of the beast, the worst slums of Venezuela, are taught to play instruments in the now famed Venezuelan Youth Orchestra. Instead of being left on the streets to a life of hopelessness, with nothing left to lose and at great risk, these young ones are given worth, substance and salvation.
Or the case of my father, who was sired by a garment worker with a third grade education and survived thanks to the Great Depression welfare programs. When dad graduated from high school, his father found him looking at for hire ads in the newspaper. His father informed him that he was not going to work but rather go college, so that he could be "...better than him." Years later, when my father had proved his mettle in the business world, he was the glow in his dad's eyes.
For better or worse, in all the above cases it was a human being(s) whose decisions sealed a fate or fates, whose ripples spread outward into unfathomable dimensions. Every action counts, large and small...