Could've, would've, should've...it's too late now!!!
Also don't pin this on Jooin, he never would have approached Sunbae after that drunken night if he didn't purposefully flirt with him since he realised Jooin had an attraction to him. But ok, that was Jooin's fault, he got into something expecting more than Sunbae was willing to give but now he's out of it. Doing the right thing, ending it becasue he is not happy with the way it is... it's better than to keep jerking him around coz as we saw in this chapter, even when you are clear with him about what u decide, he will still lash out in the worst of ways. F* him!!!