So I'll share a theory that comes directly from that french youtube video : along with my speculations. There he explain that Estarossa is Mael, but Gowther changed his memory with the memories of Zeldris, Meliodas and Elizabeth (personally I don't think that Elizabeth memories were put). That's why Estarossa doesn't look like none of his brother (tall, beard, grey hair, ...). The part where Estarossa admired Meliodas is the memories of Zeldris admiring his older brother, Meliodas saying he'll try to arange things between Estarossa and Elizabeth could be Zeldris memories but with aranging Zeldris and Gelda. Meliodas memories could be the part where Meliodas was always with Elizabeth and Estarossa developed love for her with that. Estarossa wasn't affected by GrayRoad commandment when he killed Meliodas, ... There is plenty of clues, post a comment if you saw one ! Let's not forget everybody else, to stop this Holy War, Gowther altered the memory of everybody concerning Mael, and created Estarossa, that's why the Demon king, Meliodas, Elizabeth couldn't remember properly Estarossa. We don't even know how look Mael (he just have long gray hair, Estarossa maybe have long hair but he cut it to look like Meliodas). Gowther always looking guilty talking about Estarossa and Mael. https://imgur.com/rN5Zov0 I think if he did that was to restore the balance between the two clans again, by "killing" the strongest archangel and adding a new commandment, the balance was restored but the war was still raging and it needed to end in one way or another (this is where the goddess were sacrified). Let me think what you think about this and let's debate together ! And sorry for my poor writing skills ^^'