Ugh, it's going to be yet another self fulfilling prophecy. Imagine for once, there was a story where a prophecy is foretold and the person in question is like, nah fortune teller, you're full of shit, there's no way my lovely son is going to genocide humanity. 35 years later, oh god, oh jeez everyone is dead, why didn't I listen to that fortune teller! Granted, if the laws of that story's universe dictates that the future can be changed and you were to just off the kid right at the start after hearing that prophecy then there wouldn't even be a story. "The Stereotypical Life of a Reincarnated Lady" doesn't count because MC is too braindead to understand the really obvious warning the foreteller gave her before. Character induced stupidity, why? Because the plot demands it of course! Also it's funny.
But no, instead, this story structure IS ALWAYS, what's that!? He's going to do what? GUARDS GUARDS! Throw him into the deepest darkest dungeon we have, starve him, whip him, torture him! The child, bewildered and saddened by the betrayal of his supposedly loving parents, escapes and plots his revenge which leads to the genocide of humanity, fulfilling the prophecy.