I think Doyun had a big crush on him. Teahan constantly assuming he's an alpha probably made him feel like he would never be looked at like an omega and would never be able to have that alpha omega relationship. With his family and his doctor basically telling him he was defective for years and the guy he liked calling him an alpha probably killed his self esteem. I can see how that would breed resentment.
I was thinking something like that too. The idea of Doyun getting offended of being 'misgendered' by an Alpha is so cute to me (n˘v˘n)
I like your explanation. I've being wondering about why Doyun always said he hates Taehan everytime.
This is the only explanation I've read which makes complete sense as to why Doyun is so hung up about hating Taehan.
Why is Doyun so pressed about Taehan mistaking him for an alpha? He knows he looks more like an alpha than an omega.