Poor Junwoo, he was so blinded by his clingy, needy feelings towards Hamin that he was completely blindsided and outplayed by Hamin and Taeyang. It's because he's busy trying to "decide what's best for everyone" that he doesn't inform her that her "best friend" is a sack of human shit. If Junwoo just cared a little bit less about all of this, he could have seen the obvious; No one wants to be friends with shitty people, and even fewer people want to have that kind of a friendship with a shitty person "protected" by being lied to.
If I had a friend who was secretly a serial murderer, and anybody told me the truth, I wouldn't be angry at being told, or at the person who told me, or be busy crying over my lost friendship. I would be running for my fucking life from a murderer, and thanking god that somebody gave enough of a shit to tell me the truth.
Jesus, Junwoo really needs to detach himself a bit and grow some common sense.