It is precisely becasue we know that humans have these violent instincts that we want a government that is strong enough to punish the initiation of force and bring justice, but weak enough to initiate force against its own citizens.
This is a very delicate balance.
Both anarchists on one extreme, and socialdemocrats on the other, claim this to be unattainable.
Ayn Rand was a romantic in its best sense and meaning. She was not an utopian. She challenged us to build an ideal society out of reality and reason.That's what makes her not an utopian, but a romantic thinker.
It is possible, using reason, to think out a government strong enough to prevent "Lord of the Flies" situations and still weak enough to avoid becoming a parental figure for all aspects of our lives.
Possible doesn't mean easy.