I believe YoonJae is the most hypocritical of them both because he did whatever people told him to do while feigning to be concerned about the morality of his actions.
Sejoon on the contrary is in his own reality. Although he did act insensitive , he was kind of... naive ? I think he truly expected YoonJae to be his friend and to lean on him. Yet YoonJae never offered his attention to him and kept only receiving help and money from Sejoon.