Hmmm....it is not very well explained, better read the novel but he sees her as a traitor who even rejected her own parents and married her cousins 's killer so he thinks she is really stupid but the OG Rosé was a real badass in reality, she fooled everyone....he is mysoginist but he is also young and lost all his friends, family etc to the Emperor's dog (Eric father and others) and Rose was famous for being a dumb in love with their enemy even threatening Eric to marry him...

The only type of enemies to lovers I don't like is personalities like the butler. There's certain differences or squabbles characters can break through with each other which is why it's great to see since it's usually takes communication and growth. But this mf was actually, no actually being a misogynist towards her. He knew she didn't even like his boss and it's was only business. Was talking shit at her within hearing distance, lied about a made up fiance that wasn't a misunderstanding, he knew it was the prince, but lied just to intimidate her. Acting like he was his masters human chastity belt.
Secret theory: it's like how some people say there is this weird psychological thing where certain men are threatened initially by women they see as attractive. I think he liked he off the rip in his deep unknowing subconscious.