The guards mention that all of the deaths occurred within a block or two of your current location. They are nearing the end of their shift and dreading the reports that will keep them late so they are not very talkative. They suggest asking the locals what they know. (Gather Info check)
Meanwhile, Daphne notices that the attackers have uniquely large bare feet with some dried mud (or maybe just street filth) caked on. It's possible they may have left some sort of trail. (Survival check with Track feat)
Arzag does something that is unthinkable and horrifying to Burin....
An elderly olven man runs a herb stand near the stage. Too old to run, he had hidden behind his wagon when the trouble broke out. Arzag correctly guesses that he saw the whole battle and may have more to say, so he strikes up a conversation.
The man has lived in Southspur the last 20 years. His people left the area when Brindinford was settled but he had never met men before and was intrigued with the prospect, so he decided to stay.
He worked for several decades doing odd jobs - scouting, guarding caravans, even teaching some of the men to live for weeks at a time off the land.
When he grew too old for physical labor, he turned to providing herbs that eased the odor of house and body. Some made the stews taste better too.
When steered back to the topic of the murders he relates a few key points:
All of the deaths occurred in the immediate area, within just a block or two.
no apparent connection exists between the people killed, other than the location. They included street urchins, relatively successful business owners, street toughs, and helpless innocents. Sometimes individuals were killed in dark alleys, others times entire families were slaughtered in their homes.
This is the first time he has seen a murder first hand. There are plenty of locations in the area where a band of those monsters could hide - abandoned warehouses, deserted homes, those sort of places.
Arzag thanks the man for his help and then, gesturing to his companions, says, "You know, we might be able to use a man of your skills. Those creatures seem to have left some tracks and we could use someone to help us track them. We could pay, of course."