I think when he started d discussion he was thinking mostly abt vansh but when Doc started saying that addict can go anything and can cause real harm, that's when Veer started becoming concerned abt Ichcha and realized he has done mistake
Right When he told "Galti Kar Di" I think it meant he should have convinced Ichcha somehow to refrain from this marriage.....I think this would be the reason why he goes back to Mumbai...