Only thing I can say is that dominating the will of an intelligent humanoid long term (enslaving them)is an evil act. In fact, i'd bet a paladin of Iomedae would have a huge problem with it. Also, since Desna is one of the major/popular worshiped gods in the town the people would be very displeased once they found out what was happening ( a DC 15 sense motive check per the spell).