Yu Yu Hakusho1992

Koenma:
[about Sensui] His universe was cut and dry, black and white, demons were evil so he had to destroy them. And that's when he saw what changed him.

Kurama:
What was it?

Koenma:
Gray. Truckloads of gray. I'd given him an urgent case... if only I'd known the results.

Kazuma Kuwabara:
What kind of case.

Koenma:
I had ordered him to seal up the tunnel between the human and the demon world.

Yusuke Urameshi:
What?

Kazuma Kuwabara:
You mean it's the exact same thing we're doin' now?

Koenma:
The circumstances were different.

Kazuma Kuwabara:
Really? How?

Koenma:
That opening was only ten meters wide. There was no threat of expansion, but it was stable, allowing lower class demons to pass. It took eight psychics, the strongest they thought existed, to maintain it, but it wasn't free passage for the demons. It was a trap organized by the young Black Black Club. They captured demons as they squeezed through and sold them at enormous profit.

Yusuke Urameshi:
Black Black Club rings a few bells.

Kazuma Kuwabara:
D'hose are the dirt bags who watched us while we rescued Yukina.

Koenma:
That's right, and one up-and-comer had just worked his way into the group by his talent at capturing demons - Sakyo.

Kurama:
Quite the turn.

Yusuke Urameshi:
Sakyo? You mean he was already startin' stuff while I was gettin' kicked out of kindergarten?

Koenma:
One way to look at it, yeah. Today there's an entire culture of demon bodyguards for corporate crooks. A multi-billion black market industry. Sakyo began it all.