Blade of the Immortal

Manji is a samurai who cannot die. Worms in his blood, planted by an old nun, knit his wounds back together no matter how badly he is cut, and he has killed enough people that he wants the curse gone. To earn his death he agrees to help Rin, a teenage girl, kill the swordsmen who slaughtered her parents and run a school built on a new philosophy of the blade. Across thirty volumes Samura turns this revenge setup into a long meditation on vengeance, mercy, and whether killing the killers fixes anything. It is bloody and morally tangled, and almost nobody in it is simply a villain.