From what I gather he was the main vocal that left because he was no longer able to perform. For some reason he got really sensitive to bright lights,as if you're on stage. As he became unable to perform he left the group and then the group fell apart I believe abd disbanded like a year later. Not sure of the drugging incident (which could have been in order to rape him) was the trigger for his photosensitivity, but he went through something and is now chronically depressed.
It could also just be one of those cases where people get drugged in order to make them seek out the drug again and get them addicted in order to gain customers.

I am so confused with the translation (no wonder since the earlier chapters are AI trans, not the Moonflower). Is Jaeyoung the member that left the group? The former main vocalist? I thought it was a member that starts with B?
- we have drugging incident
- stage accident??