Thus why they have not built their temple on the ruins, they can't risk losing the manpower going into the crypts and dying with the rebellion chomping on their bits so send mercs, if they succeed you look like the hero as you sent them to save the town. If they fail, you lost nothing as you didn't pay them or anything and your still in the same position as before. And saying Morn seems to be the leader of the rebels, at least in this area it makes sense he would assume that whatever is causing his troops trouble is because of Morn because why wouldn't it be when Morn has been doing it for so long it simply makes sense. Add in turning the public against Morn by putting the blame on him allows him to keep minor control of the town up until the point to many fall asleep at which point it would either become Morn storms the city "freeing" the people from Zhent rule (with or without a cure), the zhents leave the town before they lose more (unlikely), the town turns against Morn as more people keep falling asleep and never waking, or the people rally behind Morn.

The Zhents having a defensible position are no doubt trying to hold out for the option where the town rallies behind them as they can now use the townsfolk as shields proving Morn a bigger villain if he harms them, the people he says he is fighting to protect. All the while he is providing safety for the people in his little fortress should things degrade that far. Not a bad plan if I say so myself. Mind you probally not his motives, but from that little bit of text this is the line of thinking I assume Delmuth would follow for the Constable/Zhents at this time.