At fault for what? He just chose to stay with him until he knew for sure there was no chance. No victims this round, he's just playing the long game trying to see if he can make a play.
Alex already told him he doesn't like relationships he is the one clinging so if he gets hurt or if Alex continues to be a jerk towards him it's not Alex's fault because he clearly said let's end it if you're in love with me.
He's acting like a jerk towards DG so he wouldn't get the wrong idea. He does not want to lead DG on like what that hyung he used to love did.
Yes and it's a totally reasonable thing for someone who says he doesn't like relationships to literally never get in a relationship again, almost everyone who has ever had a bad breakup has said that, they get over it. He knows this, he's just looking for an in, or at least an explanation after all the shit he already put him through out of jealousy. Don't blame him a bit.
DG is the one at fault here Alex sure is a jerk but at least he gave DG a choice and DG chose to be his toy even when it's hurting him.