Never mind free will vs randomness, surely the fact of the matter is that human beings are ultimately predictable, at least on an emergent scale. If we have free will, are we still confined to an extremely narrow path based on our addiction to rationality? In terms of a larger scale, is the courseof human history pretty much guaranteed based on our biology? Or can the randomness/variation in rationality (if it exists) inherent in an individual construct singular events that have an effect larg enough to change the course of the future dramatically?
The way I see it, the catch is that the variation in rationality becomes much less of a proportion of total rationality as more decisions are made by an individual - some decisions will suffer more "irrationality" than others, depending on the situation. And the world-changing events, by far and large, aren't "singular" per se - they are constructed of a vast series of small events and decisions, thus any trace of irrationality leading to such an event is swamped through scale.
Wars are a great example. People don't just "decide" to invade. The reasons behind invasions are insanely complicated, often stretching across hundreds of countries, and thousands of years. I don't really see the possibility that Hitler was a bad guy just because he decided not to have tea one morning.