Maybe I'll make this as a proper post with manga caps at some point but watching Reigen fall apart over the course of the manga!Asagiri basement scene is so brutal. he starts off completely in control, pleased with himself because he dominated everyone else at the rock-paper-scissors and saw through Mogami's ruse when none of the other psychics did, completely in his element.
And when Mogami reveals himself, it's dramatic, it's scary, but it's just another evil spirit, right? They've dealt with those before! And after Mogami stalls for a while, letting the other psychics try their stupid attempts at exorcism, he gets another win when his jumping knee strike is able to knock Mogami's spirit out of Jodo. At this point things are scary, but his mask is still up. He still has the veneer of control.
But then Mob's exorcism fails. Reigen didn't know this was possible; Mob has always been able to do everything he asked of him. This is the point where he realizes what kind of danger they're actually in, when his mask starts slipping.
And then the other psychics hatch their murderous plan, and Reigen doesn't even try to talk them out of it. Reigen, who has never lost a proper argument in his life, immediately knows that these people cannot be reasoned with. He's frozen, because none of his tools work anymore. Because they took something he did, using a physical attack to knock a spirit out of a body, and extrapolated it to the extreme, and now they're trying to kill a teenage girl.
Reigen believes in society. Reigen believes in the duty of adults to protect children and the duty of adults to act like adults. As long as people believe in responsibility, even if they're shirking it at the moment, as long as they believe in the tenets of society, Reigen can talk his way out of anything. But here he's speechless.
It's Shinra who tries, who steps up and says no even though it's obvious that he can't succeed, who puts his body in the way of this horrific plan and as a result takes a blow that would have been fatal if mp100 was just a tiny bit less kind.
And Reigen can't do anything.
And then it's after Shinra goes down that he slams his mask back on, because he has a new goal. He can't salvage this situation and he can't save himself and he couldn't save Shinra, but maybe he can save his kid. And so he says hey Mob, the thing you need to do is go and get help. It's going to be fine. Don't worry about the rest of us, Mob. Don't worry about me.