I'm at a loss as to what Sihoo did that was so wrong to Myeonghan.
Regarding Sungbeom, I assume that because Sihoo won't be honest and say that he isn't interested in him romantically, Sungbeom believes that he is leading him on or would rather leave things ambiguous so Sihoo doesn't get hurt. However, he shouldn't have to do that? Sihoo has made it pretty clear at this point that he likes Myeonghan and is on his side. Sure, I feel bad that Sungbeom has a bad family and clearly has his own issues- but that really is not Sihoo's fault and he is not the bad guy for not reciprocating his feelings or accepting the abuse of the guys he likes.