so my theory is that the old demon king adopted the mc because he probably knew that the guy is a hero? because it wont make sense why the old demon king would even bother raising a human in the first place. plus, its safer to raise the hero and defeat the humans (since the mc dont have attachment to humans back then, it wont bother him), or just let him live so that there wont be a new hero as long as the mc is alive.
.
.
.
but the new demon king blew up all his father's plan ( ̄∇ ̄")
along the lines of that theory:
i'd guess the demon king was aware of the the old general's plan and now that dariel was dismissed because of the new general's attitude, he's just gonna wait and tell the guy, how hard he fucked up when the situation is un-salvageable just to hit home, how stupid and short sighted he was.
the old general knew that dariel would not progress (because human) so he taught dariel many skills that made him valuable to the demon king's army so they'd reasonable keep him around without any suspicion
the lie didnt last long haha ( ̄∇ ̄")