But the demons gain from the mortals presence - they get drug addicts, the walker got an army of mortals, they get a constant flow of souls. If they would kill all the mortals, for a short period they would gain many souls, but in the long term they would lose a lot of souls.
P.S. I thought about an emotion drug based prestige class. Here is the first draft.
Spoiler
The emotion narcomancer, emotion drug based caster
Yoris watched from the side as the mysterious man appeared, wearing a black hood and riding a black steed. People fleed from him in terror. He got off the horse and took off his hood, and Yoris saw something he didn`t expect –
The mysterious man was terrified.
Requirements:
At least 2 emotion related spells known.
Level 5 caster.
Be addicted to at least 1 emotion drug.
Class skills
Spellcraft (int), knoweledgd – arcane (int), craft (int), concentration (con)
Bad BAB, good will saves, bad fort and reflex.
+1 to caster level for every level taken as emotion narcomancer.
Emotion loaded spells: At first level, the narcomancer gets +1 to bypass DR and save DC while under the effects of emotion drugs or while casting emotion related spells. Those bonuses stack. In 3rd level those bonuses rise to +2, and at 5th level to +3, and still stack.
Emotion specialization: At second level, the emotion narcomancer picks a favourite emotion. Spells cast with that emotion get +1 to bypass DR and save DC, which stacks with everything.
Spontanous spell: At fourth level, once per day, the emotion narcomancer can cast 1 emotion spell of his second highest level or lower for free, which doesn`t count against his daily limit of spells.
Spontanous emotion: At fifth level, each day that the emotion narcomancer casts an emotion spell while drugged, he must do a will save in a DC of 15 + the highest level emotion spell he cast that day. The caster needs to do the save 1d4 hours after he cast the emotion spell or the second time he casts an emotion spell in the same day. If he makes the save, nothing heppens and he doesn`t need to save for the next emotion spell he casts that day (but needs to save for the one after that). If he fails, he suffers the side effect and secondery effects of the drug that causes the same emotion as the highest level emotion spell he cast that day, and rolls again. If he fails again, his emotions explode, which cause everyone within 100 feet to make a will save in the normal DC of the spell or suffer the side effect of the emotion drug which causes the same emotion.