Cassiel was the "archangel" (second in command) to Dagda, the God(Angel) of Balance (Force)
Cassiel is actually the one who came up with the idea for the compact, when he saw all the harm that the wars between the gods were causing. He wanted to put an end to it, and so convinced his master Dagda to gather the other neutral gods together to enforce a truce. They forged the Godslayer, and agreed to fight any angel, good or evil, who continued to fight in creation.
I believe it was shortly little after this that all the gods were convened at the Seven Pines to work out the details of the Compact and to sign it. Then they handed the Godslayer to the legendary Finner, and tasked the Order of the Elohim to guard their holy sites. This was the End of the Godswar and of the Age of dragons (dragons were banned by the compact).
However, the compact did not go far enough in the eyes of Cassiel. It didn't allow the gods to fight each other directly in creation, but they could still be involved. Their powers were limited to based on the sun total of the faith that mortals had in them. They were still free to wage war against eachother by proxy, having their worshipers destroy each other in their names.
Cassiel could not stand this. He had called for the Compact to protect humanity, but it had reduced to pawns in the Gods petty arguments, which his his eyes had nothing to do with their own interests of well being.
Thus he chose to fall, not for love of power like Agares and those angels he corrupted, but for disdain of it. He firmly believes that the angels should follow the example of The One, and stay completely uninvolved in the affairs of mortals. He himself, however, isn't completely uninvolved, since he feels that mortals need someone to lead them and stop them from falling prey to the petty ambitions of his fellow angels. Staying true to his principles, he refuses to use any abilities that a mere human could not, even when doing so would mean saving the lives of thousands (He wouldn't use his divine power to heal those dieing of the plague, but instead created the Ordine Medicos (the order of grigori medics) to heal diseases without relying on magic or prayer.
He also gave shelter to the Luonnatar, although he doesn't agree woth them on everything. The Luonnatar were a group of mortals who figured out the truth about the gods and The One, and began to worship the one in secret. Since they "blasphemed" both good and evil gods, they were persecuted in all societies. However, their faith in The One miraculously saved them from magic both arcane and divine. Once a group of them was captured by Order Priests, who decided to execute them by tieing them so a stake and calling own fire from heaven to consume them in the center of a crowded town square. It came, but burned only the ropes binding them, letting them walk away unharmed to the shock of all the on lookers. They found shelter among the Grigori, although Cassiel himself disapproved of them worshiping the one; he personally believes that The One doesn't want to be worshiped, and that men should rely wholly on their own strength. He will not, however, allow a minor religious difference like this to escalate into conflict.