Yu Yu Hakusho1992

[Sensui saw something he never should have seen. Koenma explains]

Koenma:
A ritualistic party they called The Feast of Human Vices. These privileged humans were toying with the demons they'd ensnared, torturing them with no motivation other than their own twisted pleasure. Sensui had sworn his life to protecting his race from demons but, here, humans were enacting cruelties a thousand times worse than anything he'd seen from demons. Black and white blurred, flipped upside-down. His mind wasn't big enough to hold the change. Sensui lived his life on the straight and narrow, but what he saw that night went against everything he knew the world to be.

Kazuma Kuwabara:
A lot like we felt when we went to Tarukane's place and saw what they were doing to Yukina.

Yusuke Urameshi:
Yeah, you're right.

Koenma:
But there is a most important difference between your reaction and his. There, in that room, Sensui fell into a rage and murdered every human at that feast.