Hulagu Khan did not execute the Abbasid leader, Caliph al-Musta'sim, immediately; he wanted him punished for his foolish defiance. He was captured and made to witness the slaughter of his citizens, the destruction of his city, and the plunder of his treasury.
Even then, a simple execution was denied him. al-Musta'sim was pulled up in a rug and laid on the ground for the Mongols to ride their horses over him repeatedly until his death was certain. This was considered an act of "caution" on the part of the Mongols, since they believed spilling royal blood on the ground would be offensive to the earth. (A colorful 15th century legend, pictured here, is that al-Musta'sim was imprisoned with his treasures and allowed to starve to death.)
Wassail, Persian historian who was born seven years after this event, but no doubt had access to eyewitness accounts, wrote:
They swept through the city like hungry falcons attacking a flight of doves, or like raging wolves attacking sheep, with loose reins and shameless faces, murdering and spreading terror...beds and cushions made of gold and encrusted with jewels were cut to pieces with knives and torn to shreds. Those hiding behind the veils of the great Harem were dragged...through the streets and alleys, each of them becoming a plaything...as the population died at the hands of the invaders.
Not every inhabitant of Baghdad was slaughtered, and some had an advocate among the Mongols. Hulagu had an Assyrian Christian wife, Dokuz Khatun, who begged him to spare as many Christians in the city as possible. He honored her request, and offered the royal palace to a Nestorian Christian Patriarch Mar Makikha, to be made into a cathedral.
Baghdad itself, however, took awhile to recover. Hulagu left 3000 soldiers behind to rebuild the city, but irrigation canals that had been damaged were not repaired. Agrarianism suffered, and it was many years before Baghdad once again became a great city under the Ilkhanate.
The destruction of the House of Wisdom and its contents has been disputed in recent years: there is a belief that some members of the Mongol army had more respect for learning and would have preserved the volumes for installation in other libraries. Hulagu had brought with him to Baghdad a scholar, Nasir al-Din al-Tusi, who had been in the fortress of Alamut (and some believe was the Mahdi's servant who betrayed him). There are stories at the time that al-Tusi would have saved much of the library.