@Kaiju In his world, the law of Murim was the way of the world: As a cultivator, if someone annoys you, you send them to the underworld. The weak revered the strong etc...
For a weakling that he can obliterate in a second, having to endure her disrespect and lack of awareness for so long filled him with hate towards her. I wouldn't have been surprised if he ended up killing her, if the story developed that way. He wanted to off her in various instances and was stopped by his companions time and time again. He's considerate to his allies, that's why he didn't do it.
Transforming to her was decent writing in my books. I also hate genderbend as a trope in writings, because it's used as a theme to grab readers which is a shame. But it's not the case here. At least not yet, and I hope it stays that way.