Rikidozan, the father of Japanese pro-wrestling, got straight up murdered.
Misawa's death was very dark.
The Act Yasukawa Yoshiko match that turned into a brutal shoot fight.
Genuine deaths in the dojos, too.
Not as dark, but this dude talking about his experience with the dojos can get depressing at times.
https://rionne.substack.com/ This is where people found out one of the wrestlers in Noah beat his girlfriend at the time within an inch of her life, but that has since been edited out because I guess the author didn't like being the conduit for an at attempt at cancelling him on twitter.
Inoki was an ultimate carny and New Japan nearly ended a few decades in due to him being found for money laundering.
You could say that the way Inoki cross-promoted wrestling with his MMA shows was dark in a morbidly funny way. Great wrestlers with big superstar potentials getting blown the fuck out in real MMA fights in under a minute and then having to become world champions in spite of that. Or a very manufactured story about a gambling man down on his dumps living in a car that had the ultimate underdog victory in an MMA fight and then went on to be way more pushed as a wrestler than he had ever been before despite the fact he wasn't skilled for it. Again, people underestimate just how much a carny Inoki was. There was also Ogawa, who was as big a draw as you could get back then but refused to job to anyone, ending in non decisive matches where fans throw trash into the ring. The early to mid 2000s are full of that stuff, and it's what led to one of the big defections when Muto left.
Speaking of Muto, his booking run through AJPW was a disaster. Having to get sponsorships with vibrators, of all things. That was dark too.
I want to say one of the executives/presidents of AJPW or Noah killed themselves when their company was in the dumps, but it's been long and I can't find that info right now. Maybe I dreamt that up.
And this is just the shit we know of. There's much more.