Man is a very self-interested species. Why would man be less violent, and destructive than man is - because man has his own self-interest in mind. Why wouldn't a Man in Power kill all of his subjects? Who would build his buildings, grow his food, dig into the Earth for his riches? I think many of us know precisely why Man has not killed himself off.
My view of Man is rather optimistic, IMO. The optimistic side is that Man can be tamed to a certain extent. No matter the generations that pass, Man will never be domesticated to the extent that either side of the Idealistic perspective hopes, prays, clings, pleads to aspiring.
I will offer an example of someone who I agree with in this instance, Limbaugh (I happen to be listening to him yesterday...he gives good headache). Limbaugh stated that Man's self-interest benefits all - that, when Man thinks about himself, he is actually benefiting all of society (a macro approach to the subject at hand). But what Limbaugh does not want to add to his little statement is that the self-interest of man is destructive, and that the only way to tame the despotic, tyrannical inclination of man is Laws, coercion, and force.