Not so much weird or afraid to pitch, but rather a thought that popped into my head that I want to write down for the sake of archival because I know I'd never get around to it. No other thread that really suits that purpose.

The gist of it is taking X fantasy setting and X group of naïve wannabe heroes that want to take down X Dark lord, master of the dark powers. And hey, they train and go through hardships and become pretty decent adventurers with a deep sense of justice. Having good intentions, however, doesn't always lead to god results. Throughout their journey the adventurers think they're doing good things for the world, even despite some sacrifices that need to be made. It starts with small stuff, like killing a giant predator bird near a village that was threatening the safety. Then they notice eggs in their nest. The first thing that comes to mind is to crush those eggs or kill any hatchlings there so no more of these birds could grow and threaten the town.

Continue on into larger issues that are a bit more world shaking, like removing some kind of crystal that gives life to a forest because they'd need it's power to defeat the dark lord, but they damn the local population (elves or something) in doing so. Or retrieving artifacts from ancient tombs, but having to tear the place down to do it and making ancient technology lost forever. By the time they'd reach the dark lord, said lord is almost depressed to see this band of heroes. Still, they fight. The heroes win, naturally, but it turns out that the power of darkness wasn't only inside the lord. The adventurers became so strong because their own heart became blackened in the journey because of all the things they needed to do to get here in the first place, and unwillingly they become the new dark lords with the entire world hating them. The dark lord they defeated laments his own fate, as he'd gone through the same thing and the cycle continues.

TLDR; Die a hero or live long enough to see yourself become the villain, even though you thought you were still being a hero.