It is not about which motive or reason is better. Its about what it the root of the problem. At this point he mostly leaves the MC only and does not bother her. The problem is Youko at this point and her feelings with Maki. Why is he detestable? that is a strong word to use for Ketia. Rude and brash of course but at the moment I will not use such a word. I am only stating you cant blame Keita for everything. At this point we are only going to go in circles so I will say we agree to disagree.
Yes, the problem is obviously Youko and her feelings, but that does not somehow absolve Keita of being an asshole who exacerbated the situation with his bullying. He is detestable, in my view, because his response to being told his bullying made someone cry is to call them weak and then refuse to stop, but that's not really what I meant. Youko considers him detestable (at least she did) which is part of her motivation.
And I've never said he caused every problem, only that he's an asshole who only lightened up on bullying someone when he thought he caused a problem with Maki, who, as others have pointed out, seems to have forgotten the whole thing about getting him to stop calling her Imoko. So, at this point, Youko's sympathetic, Keita's an asshole who everyone loves for some reason, and Maki is somewhat sympathetic but loses a good amount by being attracted to someone who bullies her best friend and doesn't stand up to him about it.