I personally place most of the fault on Brian's part, although saying "A is guilty and poor B had to suffer!" is wrong to me. He of course had a lot to deal with and he wasn't ready for it, but just a little communication could have salvaged so much. Then again, being jealous of someone probably wasn't something he was used to and it shocked him how easy to hurt he actually is.
Justin's insecurity was absolutely heartbreaking. I think pre-bashing!Justin would have thrown the hustler out or whacked Brian over the head for making that love-bashing comment (I certainly would have!) but now he's just so uncertain of everything, it hurts to watch.