I need to stop overthinking
Heyyyy nice!! I read the originals and after finding your comment I read it again for the third time. I can't find any hint that the demon was stalking the village since Fin spread the potion but your theory makes a lot of sense. This whole story would've been SO much better if the demon was summoned as a low level "kid" and was there to witness Finn's abuse and neglect; it would give us solid grounds to hope he actually had at least some kindness and sympathy for Finn. I think the narrative points to the demon just being randomly attracted to the "foul stench" he came across and not that he was actively hunting the townspeople but I'm now forcing my brain to mash the canon with your theory!! I wish this could go on as a series and watch Finn's corruption while the demon grows fonder of him; your theory adds a lot more layers to exploit. Thank you for making this more interesting!
I think the demon has the power to heightened human's emotion.
Villagers - Lust, Helplessness (during the massacre)
MC - Pity, Regret (during massacre) Rage
And the Demon is not that weak to be imprisoned, the last scene showed us that he can free himself easily.
Which leads me to the conclusion that the goal was to corrupt the MC who has a pure heart.
So my theory was,
A) when the younger MC sprayed that potion in the fields, this demon was also summoned. At that time, his power is not that great and can only influence one human at the time.
* that old man with a gun who's extremely angry and pointed a gun to the younger MC, while the other man was trying to dissuade him*
Now that the demon is powerful enough, he set this stage to manipulate the MC.
B) MC is somehow "special". I based this from the fact that he's too pure hearted and way the magician reacted after the villagers found out his schemes. The way he easily threw the mom out of the bus but adamant to let the child to live and not leave the town.
But the sure thing was the demon coaxed the MC to tell him his secrets, his regrets, flirt with him, make him the MC's special someone, open his eyes on how shitty the villagers was and heightened his rage to commit the massacre.