Demon Slayer: Starting with Negative Merit, I Save Everyone by Taking a Beating

I transmigrated into the body of Kaigaku? I awakened a Black-Hearted System, and I'm at -5,000 merit from the start? Work hard to pay off the debt? What kind of ridiculous plot is this? Give me back my cheat-powered life!

[Lots of Slice-of-Life] + [Will Change the Original Plot] + [Black-Hearted System] + [Happy Ending] + [Original Couples Unchanged] + [Somewhat Comedic] + [Not a Power Fantasy]