First of all, Xiao Zhan didn't treat Wang Yibo badly. Being friendly with other people, like he always is, is not treating his friend or whatever badly. If you are talking about Xiao Zhan playfully hitting Wang Yibo, you obviously haven't seen other BTS videos of them - Wang Yibo was also hitting Xiao Zhan all the time. Or are you talking about that one time when Xiao Zhan got a bit annoyed with Wang Yibo and told him to shut up because he was feeling unwell and his stomach was hurting, but Wang Yibo was continually pestering him (yes, with good intentions, but still)? After that Xiao Zhan tried to lift his mood and even apologised, even though his stomach was still hurting, which is more than most people in that situation would have done. Many people, when feeling unwell, would just tell everyone to leave them in piece and that would be it - no apologies, no going out of their way to improve the mood. So this is definitely not something that you can hold against Xiao Zhan either. Secondly, even if Wang Yibo was smitten with Xiao Zhan, Xiao Zhan didn't owe him anything, but he actually looked smitten too at times.