The destruction of Hatra
Hatra was not as old as Nimrud or some other cities of the primordial land of Mesopotamia. It was of the Classical period, specifically what is called the "Hellenistic" age. After the premature death of Alexander the Great in 323 BC, the lands he had conquered (basically the original Persian Empire plus Greece) fragmented into kingdoms ruled by his generals and their descendants. Alexander had dreamed of bringing together the Greeks, Persians, and other advanced peoples of the Classical world in a shared civilization under a unified state, and the Hellenistic successor kingdoms offer some hints of what the world he was trying to build might have looked like. Hatra was founded in the third or second century BC under one of the Hellenistic dynasties, the Seleucids, and the main structures that survived into modern times show an interesting mix of Greek and Middle Eastern influences:
ISIS is destroying pre-Islamic relics and non-Islamic sites throughout the territory it controls because it wants to purge the memory, and thus the identity, of that territory's people of anything other than Islam. In the case of Hatra the destruction is especially symbolic because the society that built Hatra was explicitly eclectic. The reason why the buildings shown above probably no longer exist is that they challenged the world-view of people whose minds are too simple and barren to feel comfortable in anything but an absolute monoculture.
After the Classical civilization was destroyed, the vultures of Christianity and Islam cut its corpse into halves, making the Mediterranean Sea -- the center of the Classical world -- into a militarized frontier dividing the Middle East and North Africa from Europe. Each of the two conquering Abrahamic religions set out to become the sole legitimate identity of its half of the territory, purging (rather sporadically and incompletely, thank goodness) people and ideas representing the greatness that had been before. ISIS merely represents this destructive puritanism taken to an extreme. Not only people and ideas must be eradicated, but even the buildings and art of the past, which potentially remind today's Islamized populations of what their ancestors really were.