TBH I don't really doubt estarosa's origin. He is the demon king's meek son who was given a commandment and lost his sanity. He was given the commandment of love because his father, to whatever capacity the demon king can, loved him. Estarosa being a meek child unable to hurt a fly would be ideal for the commandment. But there was a side effect and estarosa lost his mind. Enough that even then he simply couldn't feel like a normal being and properly hating or loving anything became impossible. Estarosa became a psychopath obsessed with meliodas. And thus even though he became insane and even evil he simply could not fall prey to his commandment. He was lost as a potential demon king candidate but in turn became a particularly powerful demon still.
And with that in mind, I don't think estarosa estarosa actually was as pathetic or talentless as monspiet suggested. Estarosa was simply too kind to wield darkness which would naturally hamper his development. Once he awakened his darkness of course he was ruined. Estarosa still became one of the stronger commandments and his PL was second only to zeldris among them. After meliodas defected of course. It is also plausible that even growing stronger would warp estarosa's mind even more. It seems likely that what enables demons to access greater demon transformations is a combination of skill and power. But that equates to basically more darkness. Which would further ruin estarosa's mind. Anyways, that is just a theory....
I wonder if it was a possibility to simply steal estarosa's emotions in contrast to placing a commandment within him. Stealing meliodas' emotions apparently makes his darkness stronger after all. So if estarosa, who was too kind, had gone through something like that perhaps he wouldn't have lost his sanity... It'd be interesting if it turns out this did not occur to the demon king back then.
What is a mystery to me though is the story with mael. Clearly mael should be substantially more powerful than estarosa. However estarosa killed him.... And something about mael seem to have linked estarosa to elizabeth. Personally I think estarosa ate mael's soul after killing him and that further added to his insanity. Still, that does not explain how mael lost.... I am thinking there is a chance that mael actually didn't have his grace when he fought estarosa. And perhaps he was a mostly physical fighter, which makes sense with sunshine. And even with a greater PL than estarosa if most of his skills are physical he is vulnerable to estarosa's brand of full counter. As to why mael would not have his grace, it is hard to tell. But it would explain why he lost to escanor and how it ended up in the hands of a human. Not fully but if the grace is lost even before mael's death then it makes sense the grace could end up at a random place rather than remain in his corpse. Perhaps his grace was related to papa gowther's plan or the SG's current unknown fate. The demon king was sealed so it would make sense the SG is sealed as well. And sunshine could have perhaps provided the power for that somehow.
