i have found evidence to support your theory. Very striking evidence to me.
My first observation comes from the battle dialogue between Alcryst and Fell!Alcryst.
Fell!Alcryst is telling the “illusion” of himself to stop projecting his weakness onto him. To me, this implies that cocky Fell!Alcryst has the same doubts as his normal counterpart, but has suppressed them to the point of holding deep contempt for others, especially his brother. Pretty much he traded his self-deprecation for more confidence, but traded his kindness for more jealousy in the process.
My point is reinforced by this, because Alcryst fails to point out how obviously a jerk Fell!Alcryst is. There are multiple reasons for this, the first being that Alcryst really does think like Fell!Alcryst. However, Alcryst is well-aware of the way he thinks. One of Alcryst’s most apparent traits that somehow people fail to notice is his disdain for enemies. It even says that directly in the ally notebook!
So, Alcryst is recognizing that his doppelgänger is more confident, and wishes he was like him.
My second observation comes from Alcryst’s support with Fogado, when he is talking about his past. This tells us directly how Alcryst resisted his jealousy, and how Fell!Alcryst could have grown so envious.
Fogado asks Alcryst why he didn’t demand praise and attention when he was getting none. Alcryst says that he would never allow himself to think like that, because he admires and respects his brother, putting his care for others in full display. Look back at that ally notebook page, it says he likes people, does it not? He is told by multiple people that he’s very good at pointing out the qualities of others that they might not even notice!
Because Alcryst has trained himself to only find qualities in others and not himself, and receiving little to no words of affirmation, he naturally became the self-deprecating but kind prince we all know and love.
Then Fogado directly talks about jealousy. Alcryst never hated Diamant. He would never allow himself to. So what if he did hate Diamant, resented him because only he was praised? What would he be like?
Well, we have the answer.
The final thing to note is that Fell!Alcryst is not completely just arrogant and self-centered. It’s more that he wants to prove that he’s better than Diamant. His battle expression is more of an “Oh, now’s my chance to prove I’m better than these losers!”, but he takes that to an extreme (he REALLY wants to kill people).
He is also not completely merciless and bloodthirsty. He is still a human, and cares about people, showed in his battle dialogue with Lapis and Citrinne.
While confidence has taken over Fell!Alcryst, he still has lingering self-doubts. And while jealously seizes him, there is still a part of him that cares deeply about his friends.
Fell!Alcryst is still willing to protect those who are close to him.
GODDDDD THE WRITING IS ACTUALLY SO FUCKING PEAK, I LOVE SEEING INVERSES OF CHARACTERS IN ORDER TO SEE WHAT THE ORIGINAL HIDES RAAAAAAAGH