@squinney,
I believe history is made by individuals, but mostly by inter-twining social, cultural and economic forces.
Back to my points:
-There was Mussolini before Hitler.
Italy, Germany and Japan were all nations that arrived relatively late (in comparison to England, France and the US) to industrial capitalism. Italy and Germany also arrived late to national unity. The sole forces of the market were insufficient to make them competitive in a world scale, so they needed a strong State... but not a socialist State, like it happened in Russia, Fascism was a movement to organize the social forces around an authoritarian State, with both centralized capitalism and expansionism, which all three countries needed for their national purposes. It also meant crushing the left-wing organizations: this was done through thug party groups in Italy and Germany and through the army in Japan.
Had Hitler not been born, some other extreme right movement would have grown in Germany (in 1919, it almost became a Socialist republic, with the uprisings in Kiel, led by Karl Liebnecht and Rosa Luxembourg), would have been financed by industrialists such as the Krupps, would have risen to power, repudiated the Versailles Treaty, and make expansionist moves. It would most probably also be anti-semite, because that was a shrewd way of both distracting the populace anger and breaking the spine of the workers' movement.
-The US had to get into WWII
At one moment of that conflict, Britain alone was defending the world's democracies against Fascism. On the Eastern front, Stalin had moved most of the capital goods factory inland (out of Ukraine and Belarus), because he was expecting the Germans to repudiate the Ribbentrop-Molotov treaty. Had Britain fallen, the battle would have been between two branches of authoritarianism, and there would have been little space for the US to maneuvre, wanting both to sustain the capitalist system and the basic freedoms.
During those years, the American right wing was promoting neutrality... which meant promoting a Nazi victory in Europe; while the progressives were for joining the war. Pearl Harbor was a perfect pretext. Had there not been such a bombing, some other event would have moved the US into war.
Mankind won with the victory of the democracies.
On slavery
-Just one question. How much of the US industrial revolution was financed -via North/South trade- by 600 billion hours of unpaid work? (or 300 billion, if we suppose that the maintenance cost of one field hand were probably half the value of the revenue the master received from the slave's labor).
... And on Christianism
-Christianity was at first the slaves' religion, it served a social and even organizational and rebellious purpose. Most of its attractive things come from that origin.
Things changed when the primitive church-movement transformed into the church of power-institution. So maybe the "catastrophe" was one of the first councils, perhaps in Nicaea, 325 AD, when Chistianity and temporal power melt.