I could be remembering demon lore/ bible demons incorrectly, but here's the ramble anyways:
Isn't there an interpretation where Lucifer fell because he came to believe humans were equal to angels? That was blasphemous and thus cast out-
So is MC corrupting Luke simply by hanging around him because he genuinely likes them as a person? He is constantly equating MC's words/actions to the way angels act, in his opinion.
Everyone thinks it's adorable, Luke clinging to MC and seeing them as an older sibling, or even a parental figure. Great! Silly little Luke! Regardless, the exchange program worked, he was exposed to demons and humans and now he understands the world a bit better. Just not how they wanted him to understand it.
Luke overhears a group of angels passively dismissing humans and their silly actions and passions; how easy it is for humans to fall for petty ambitions such as money or attention. These conversations are common in the celestial realm, and while Luke used to jump in and bash humans along with the adults- eager to prove his knowledge and superiority to the easily corrupted mortals- this time he can only let out a frustrated sigh as memories of silly antics with you come to mind instead of the images of greedy humans that used to come to mind. Luke can't help but feel a little embarassed, knowing exactly how the adults would grimace at him and send him off with a side eye if they knew, but he's also mad at himself, for ever feeling embarassed for having happy memories with MC.
As he avoids more of these conversations, resentment starts to plant itself in his heart. He'd been so eager to talk about you and all the antics he'd seen in the Devildom when he first came back, but it never ended well. The other angels would make unhanded comments about the brothers and MC, saying how they'd never be caught in those messes and how disgraceful that must have been, offering Luke sympathetic hand pats and pity sighs for his distress. But he hadn't been distressed, and he couldn't make them understand that; yeah, some of the stories sounded bad out of context but it really wasn't that bad, if only they could get to know MC they'd understand! Only Simeon would smile and laugh at his stories, offering his side of things only when others weren't around.
The other angels praised Luke's baking. He'd always been a good baker but now his pastries were unmatched. An unexpected benegit from the program- he must have been so scared and isolated in a place like that, of course he had a year to do nothing but hone his craft! But nasty little rumors start to spread, better traveled angels recognize the techniques used and it becomes undeniable the child has been learning directly from demons.
"Do you think it's true? That he baked for the demon prince and lords directly?"
"He must have been forced to do it, such a tiny thing couldn't put up much of a fight if Simeon was elsewhere with his former friend"
A council meeting is held late at night, only the highest of the elders allowed to attend. There is only one thing to discuss, the child who was bordering on blasphemy. He isn't as attentive or social, seeming much more withdrawn than before he was sent to the program. He still performs his duties and attends ceremonies, as all angels do, but he is seldom found at the events afterwards, seen sneaking off to stroll the gardens or retreating to his private kitchen again to bake until he's called on again. Even though they had to make sure only the highest authorities knew of this meeting, no one in the councik was worried on how to proceed, such behavior was easily amendable.
At the next monthly ceremony, Luke is presented with his own holy adornments in honor of his hard work and dedication, 2 identical cuff bracelets and and a matching brooch. In the sea of gentle applause, no one cares to notice the only person watching with a strained expression, not that he could do anything now if he wanted to. He thought he'd taken enough attention off of Luke after coming back, Simeon had known exactly what was expected of him after returning from the Devildom. He never spoke too lovingly of his time there, and was always sure to put in sly remarks against his old friend and the other royals of the Devildom as he was expected to do. Simeon can't help but wonder if there was anything else he could have done to prevent this, a lump settling into his throat as the cuffs were clasped shut on to Luke's wrists.
MC's in the Devildom, pacing the halls of RAD. They'd heard that Luke was finally going to visit again, and even if it was only for official buisness, MC hoped they could convince him to stay for a quick cuo of tea and some of his favorite dessert as Madam Screams.
MC not immedieatley recognizing the tall boy who walked past them, stiff and closed off, without so much as a nod, but that fluffly golden hair was impossible to misplace.
The tall boy turns back, still looking bored and even a bit annoyed, doesn't even crack a smile when he looks at them again.
"MC, you're still here." There's a nod of dismissal and nothing more as the angel who MC once considered a brother walks away from them.