Heya Russ. Firstly, there's very little here I disagree with you about in this thread.
However...
Can you imagine how great it would be if we all believed the same way.
Can you imagine how dreadful it would be? All the cultures and traditions and languages and religions and perceptions squashed for the desire of sameness. Unless I misinterpret your meaning here. There is such a thing as too much of a good thing. It may seem wonderful, the idea that everyone share the same belief (are you inferring religion here?) so as to avoid holy wars and the consequent innocent lives taken as a result. But I think a world possessed of one belief would be an even greater sickness on mankind than the already existing dilemma. Novels such as 1984, Do Androids Dream of Electric Sheep, Fahrenheit 457 and even the film THX1138 immediately spring to mind. I'm talking about dystopia here. Utopian agony. Sameness.
Ultimately humans behave differently because we
are different. I've never found the expression
people are people to be anything further from the truth. If humans were non-theistic animals, not in the philosophical sense of the word but if it were something which never entered our minds, rest assured we'd be waging war and killing one another over other things instead. Besides which, nine nine times out of 10 wars fought in the name of religion are a mere cover up. In the time of King Henry VIII, England was trying to separate from Rome, with religion as the ultimate explanation - at the end of the day, though, this was just a facade. It wasn't about religion any more than the war in Iraq is about "liberating Iraqis".
So whether it's religion, territory, resources or something altogether different (or something pretending it's something else), humans will behave as it is in their nature to do so and the more human among us will try and impede the less human (murderers, rapists, etc.) from atrocities. But sameness worldwide is not only an impossibility but the attempt to do so is a greater violation than the bloodiest of holy wars. Just think of all the fascinating cultures in the non-industrial world -- some of which have never seen things like television or telephones (and I'm not talking 3rd world poverty here, I refer specifically to tribes in the Amazon and elsewhere) -- and how these beautiful peoples would cease to exist all in the name of sameness. All so that we can believe the same thing. One might as well say: think how great the world would be if we all listened (and only listened) to country music, or if we all had blond hair or blue eyes, etc.