This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Blade of the Immortal
Shira torturing two Itto-ryu fighters and a prostitute from the Yukimachi.
Hyakurin's torture and rape at the hands of the Itto-ryu.
Burando's experiments with the prisoners.
Burando's ultimate creation, Nue #1, is a primitive cyborg—metal plates fused into his skin across the back of his neck to make him almost immune to decapitation, a metal grill covering his face, large twin blades bolted to each forearm, an insane Healing Factor almost on par with Manji's thanks to the bloodworms infused in his body and utterly, psychotically insane!
Shira tearing apart three Shogunate officers was bad enough, but the fourth one gets itthe worst...