@bibliophage
Spoiler!
At some point it became clear that OG Sihyeon has a sponsor (the members didn't know it's Jaeha or even the fake boyfriend relationship; they simply believed that there's a guy who's sponsoring Sihyeon's debt or someone's debt he's shouldering in exchange for "special services"—you know how beautiful Sihyeon is, right?). Even within the story, the member feeling changed from concern to frustration when Sihyeon continued this toxic relationship or business with no effort to change and even got angry at them for trying to help when it started to affect the group performance, image, and later even fandom. To be fair, OG Sihyeon is scared if his members learn he gives "special service for sponsorship" to a rich client for Jaeha's sake, they will find him disgusting. Until his death, he truly loved his member. The member's guilt is basically that they gave up trying to help him and left him alone as OG Sihyeon asked them. When they saw OG Sihyeon's dead body in the hospital bed, they thought they should have been more aggressive with him, even if it meant more fights and conflict with Sihyeon, if it meant helping Sihyeon.