I’m lost, can someone pls tell me why did he had to take the drug again and who is the g...

The drug makes the user feel an extreme state of extasis but it makes you completely dependent of the drug to "feel" happiness, if you stop taking it you can't be happy anymore (that's how Jaeyoung is currently after Jun, the blonde guy, forcibly gave him the drug). The guy in the pool was an undercover cop, Jun figured it out and killed him.

thank you so much