I think you're on the right track. Maybe offer to tell more if they swear fealty to Lamashtu. ;)
Depending on the character, you might give some teaser information on the Runewells themselves.
This early in the book, though, I simply don't see that much to add. These guys are way down the food chain - as in, may not have even heard of Mokmurian.
As you progress, there are a couple lore tables to help gauge how much to reveal (like Foxglove Manor).
Above all, I recommend keeping in mind that these NPCs are not going to warm to the PCs and may even view them with higher contempt for not striking the killing blow when they had the chance. When you're done disclosing information, tell the group that the bad guy provides nothing that hasn't already been said or lapses into a brooding silence.
Maybe give the NPC the old "cyanide pill in the hollow tooth" trick so that they take what they know to the grave.