ISIS is a response to Western cultural supremacy, and in order to combat that, it decides that everything but fundamental Islamism is to be ruthlessly destroyed by any means. This includes cultural heritage sites that don't have anything to do with the West at all other than being some of the only artifacts that remain from the origins of humanity as a whole.

It is true that the West has done worse things (the Crusades, etc) and that our current standards of warfare are artificial constructs, but by the same logic, you could argue that ethics in general acts simply as a hindrance to society.

You could argue that raping your friend's wife because you think she's hot is something that could be okay because by your standards, the suffering you cause (to your friend's wife, your friend, or both) is outweighed by the pleasure you gain from doing so.

We developed these standards of combat because the goal is to minimize suffering. We don't do those things anymore because we've come to realize that we are violating our own ethical principles in doing so.

We don't use chlorine gas to deny the enemy entry into an area. We don't allow soldiers to take quarter in civilian residences and demand supplies. We don't condone looting and court-martial soldiers for doing so, or raping, or murdering.

This is because it is our responsibility to use power responsibly. If we wanted to, we can bomb any country into the stone age and indiscriminately kill millions of people in seconds, or poison water supplies for centuries, or any number of things. We choose not to because we hold ourselves to the ideals we supposedly fight for.

The 'culture' that Sallow seeded was from the Followers' work, and none of what he had done would've been possible if he hadn't been educated by the Followers. If anything, the Followers represent a better chance of culture and history being passed on, because they actually pass it on. Caesar just takes the bits that he can use to form a cult of personality and doles those out.

"Medicine is evil and everyone should use natural remedies enforced by pain of death. Except me because I get an Auto-Doc. But not you peasants."