I was thinking about Satan’s inferiority complex and… the first thing I want to say is that he never genuinely hated Lucifer and merely felt threatened by him.
In the game, we can see how he seems to dislike, mock and even do ridiculous rebellious acts out of spite or acting petty towards the oldest but it’s never something big that could actually cause major repercussions nor affect his brothers ; it’s a way of making himself stand out, aside from his withdrawn self who constantly reads, trying to one-up that one single feat of his the knowledge that he is known for.
I would like to talk about how Satan, despite feeling like nothing itself when standing next to the oldest, probably defended Lucifer when he or his siblings weren’t around : if some low demon was disrespecting the eldest or mocking him in any way, he’d be the first to defend him and using the excuse of being part of the student council and how such behavior is inexcusable but that’s not it. He cares but he is deeply conflicted about it : it’s a constant clashing of I love and I hate, it’s never a single state and most of the time he would lash out at the older, at how everything he does is without a mistake, ridiculously perfect – but he does not disregard the effort behind, he knows better than anyone else the pressure Lucifer goes through, what is expected of him and how he demands himself to surpass those expectations. It’s both admiring and despising all he does, a bittersweet sentiment he wishes he’d simply not possess.
Though at times he does feel like he’s merely a shadow and that’s when his complex hits the hardest, when he is on his own and starts questioning whether he has any value as an individual. Does anyone notice his own efforts? How hard and difficult it’s for him? He learns but it’s never enough, is he enough? Where does Lucifer end and where does he start..? It’s easier to give into his anger, his wrath, how self-conscious he can be at times. Lashing out at the first to even stare in his direction, but he never does aside glaring since he can’t afford a scene, hiding behind a mask and feelings to a surface level.
When MC comes along and helps him realize how ( not foolish since what he felt is valid ) actually it was Satan’s cowardice and inability to look further, deeper into the big picture, was holding him back it’s a turning point. It also makes me think, did none of the brothers try to guide the youngest sibling? ( They just taught him right and wrong, the concept of feelings and that’s it? Manage and figure it out yourself? ) Or did they assume he’d figure it out? It’s probably not that, they all had their own problems and, first of all, they are demons and they are all self-centered who have their own close relationships to one another ( just like how Satan seems the closest to Asmodeus ; Beelzebub and Belphegor ; Lucifer, Mammon and Leviathan ) and probably didn’t try to communicate much, they don’t strike me as the type to get touchy-feely ( they talk about Lilith and the whole atmosphere is… ).
Anyway just because MC is helping him sort out his feelings, and now that they are gone… It doesn’t mean he magically changed his attitude and loves Lucifer. At times, he will go back to hate him, back and forth, back and forth – what’s different is that now he can stop himself from having that negativity and hating mindset, instead trying to get closer to Lucifer and try to learn all that he missed, even be his aid, be of help if possible. Small steps, he won’t launch himself and will mostly be of assistance if he is asked, it’s still a difficult progress for him since he’d also feel ashamed of the unnecessary hate he had in the past.
TL;DR.: I just think Satan put Lucifer into a pedestal and only now is learning that was not the case.