Many threads are related to the Mael is Estarossa thing but it’s often confusing, so, here’s a little summary of the main theory.
I’m not gonna theorize, except a bit in the last part, about the point of their changing minds or how this happened, even if it’s likely related to OGowther. I’ll just try to pick up the clues. It’s up to you to determine if Estarossa and Mael are really the same person or are just related since their memories “fused”.
Everything begins with the first appearance of “Estarossa” or at least, with a grown version of Meliodas created by Elizabeth’s mind. Estarossa power level is not based on his spirit, mainly on his strenght. He doesn’t have much magical attributes apart from his brute force, so, he couldn’t have infiltrated her dreams and thus, manipulate her. There is a triskelion, which is the Goddess symbol, but it’s not very important since it’s basically Meliodas armor.
Cusack mentioned Estarossa as a strange being later in the story.
Estarossa is also found lost, in the same place of Liz dream, which is, according to the theory, a place where Mael and Elizabeth went. So Elizabeth asked him if he had fought with his brother again, referring to Ludociel and Mael, not Meliodas and Estarossa.
When Estarossa is stuck in this place, being Mael basically, Estarossa loses his demon mark, same happened for Dreyfus and Fraudrin.
Estarossa met Elizabeth and both sensed something familiar vibe. (ch. 246)
Until then, perfect. But Estarossa’s seemingly childhood has arrived, a brat without the power of darkness. Meliodas seemed to cherish his little brother, why would they fight? Perhaps, it was just Mael and Ludociel since their ideals were probably different. It’s a bit strange that Estarossa ended up with Elizabeth, in a celestial place, hundreds of years before.
Ludociel implied that he wanted Elizabeth to get married with someone of the Goddess Clan, like his brother Mael, making him the futur leader of his clan.
So Estarossa’s memories aren’t necessarily false there, that’s just what Ludociel said to Mael since he really wanted that.
It should be noted that “Estarossa” wanted to end the war, since Mael praised Liz, maybe Mael was the one who wanted to end it?
Meliodas has some demons behind him, Zeldris too… but there are actually few angels statues behind Estarossa, even Ludociel on the top. There is a clear and wanted transition in the colorspread between the two clans and these statues, hinting to Estarossa odd connection with Mael.
Anedoctically, Estarossa’s hand used to kill Mael is also the one “hidden” of Mael.
Finally, we can note Estarossa had four dark wings and Mael also had four wings. Perhaps it’s nothing, perhaps it’s something.
PERSONAL PART — CONCLUSION
We learned in the last chapter that Zeldris idealized Meliodas too but it’s not enough to suggest that he also wanted to be like him or even that Estarossa also has few Zeldris memories. If that’s the case though, it would mean Estarossa has multiple memories from different characters of the cast. Zeldris being the one who felt betrayed when Meliodas left his clan, the one who admired Meliodas.
Estarossa feels the same way, he’s like Zeldris in fact, he even dresses like him. Meliodas fell in love with Elizabeth at first sight, not Estarossa. Basically, Estarossa never existed until he “killed” Mael — himself. So yeah, Mael is dead.
It’s similar to Samael, a fallen angel – sometimes depicted as one of the seven archangels, an interesting figure: good and evil. Estarossa being his “demonic” name and Mael his “angelic” one.