In either story, a fun thing to play with is making the traditional "sidekick" character have to muster their will & strength to become the true hero, when in the former idea the hero becomes corrupt.

And, because I KNOW someone will eventually mention it, the latter does sound like a halfway between Undertale: Disbelief & the Starved Eggman creepypasta, specifically the part where (spoilers until the end of the sentence) Tails confronts Eggman after Sonic's death by the mechanical claws of Furnace. It's not my fault that they're brilliant stories