I think its still to early to make assumptions, we dont know how long Sanyu knew about this. It could literally have been the night MC choose to off himself, that Sanyu found out. And the shock the MC felt when a member from his band saw him in that condition, could have been the last straw.
I have not read the novel, so I dont know, but all im saying is; we dont have enough details. (=・ω・=)
I don’t think it’s right to blame anyone who wasn’t directly involved.
Spoilers:
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The rest of the members did not know the exact reason OGmc acted that way. OG mc was traumatized and would push EVERYONE away. He would cower in fear and scream at the slightest touch. He wouldn’t even let the black haired member sleep in their shared room, so the black haired dude just accepted that he had to sleep in the living room. The pink haired member did find out, but of course OGmc was still scare and ashamed that pink haired dude saw him in that state and pushed him away again. The members all cared for him, but it was OGmc that wouldn’t tell them or let them close enough to help.
Even the boss of the company actually knew the exact details of what was happening to OGmc and literally said he could help him, but OGmc rejected the boss’ help. OGMC followed everything his toxic bf said. He took no help, chose not to tell anyone or ask/accept help. There wasn’t anything anyone could do when the victim themself doesn’t want help.

I really hate Sanyu for being a bystander. Like- he could've said something. He could've settle the tension between the members and he could've comfort the mc but nah. Im sorry but Im still pissed about that ex and it could've been prevented if the MC has someone to talk to and give him good advice or something