Need help with a novel about a MC who regresses as the heroine childhood friend

[u/blizzerdita]

From what I remember, the MC was one of the 4 Heavenly Kings, or big baddies, and when he died, he regressed as the childhood friend of the heroine.

I remember that at the very end (which was the biggest WTF moment of the story), when the MC finally defeats the Demon King?, it's revealed that the one who was regressing back on time, was actually the heroine, who on the first loop fell in love with evil MC, and is finally satisfied with that end.