Not all anime Demon Kings are inherently evil, but all of them are most certainly overpowered MCs and are not to be trifled with.

Diablo, a Primordial demon, defies norms by working tirelessly ... That Time I Got Reincarnated as a Slime is an isekai anime ...

Isekai anime often ... to undo the act. Diablo must set out into this new world with his companions to find a solution and learn to handle the position of the newly summoned demon lord.