It was a war that occurred because a small, elite group of extremely wealthy slaveholders didn't want to pay any tax. The success of the 'War of Independence' eventually led directly to other attempts to destroy the established order, like the French Revolution and the tyranny of Napoleon in France. It also led, either directly or indirectly, to the near extermination of the natives of the North American continent, to the Napoleonic Wars, the disgustingly racist and imperialist doctrine of 'Manifest Destiny', the European revolutions of 1848 (which gave us the 'Communist Manifesto', among other abhorrent tracts), and from there on things just got worse (ex. Lenin, Hitler).
Was the outcome of the US 'War of Independence' the greatest disaster of the last 500 years?
Apart from the claim that is made within the first sentence here (which is reasonably accurate, if simplistic), all else that I have outlined here is of course speculative, but nevertheless plausible. No one knows for sure what would have occurred, how history would have unfolded if the American Revolution had failed, so don't take this too seriously (and I do extend the sequence of events a little too far towards the end).
However, a case can be made that the 'War of Independence' set off a sequence of events that would prove to be disastrous for mankind.