Angels are servants of the gods, whether good or evil. Shortly after the Puncture, a fiery host of condor-winged angels descended on Zaradene demanding obeisance. They claimed to be sent by Sargonnas to commandeer the city. Seth Mase and his allies made short work of them, but could not kill them; instead, the angels retreated to whatever abyssal plane they came from. After Seth’s departure, they returned in force, enslaved Zaradene, and captured Mayor Chance. The angels, led by the Metatron of Zaradene, still hold the city now, and it was only the founding of Adastra by Seth and some other escapees that prevented the entire population from being held under the angels’ sway.
Angels, especially lesser angels like those that attacked Zaradene, are almost always unswervingly loyal to the deities they serve. They are glad to fight to their own deaths (for angels can be killed), but when the option presents itself they prefer to return to their own plane when outmatched to lick their wounds. In general, the only way to make a truce with an angel is to make a truce with its god.
Sargonnas’s agenda seems to be enslavement, not death or destruction. It’s unknown what he seeks to gain through this enslavement, and it’s also unknown what other towns might have been captured by his armies.