Harry/Uma headcanon: Though she would never admit it, one of the biggest reasons Uma hates Mal with passion is because she broke Harrys heart.
I could definitely see that, considering that Uma comes across as very loyal to what she deems as hers, crew no exception, but especially with Harry.
Harry is her eldest friend, her first mate, someone so dear to her heart that she wouldn’t hesitate to rescue him if need be, and the one person she knows she can count on to never falter or waver away from her. So, some girl coming in and breaking his heart would be unacceptable in her eyes; not to mention, that this is the same girl whom she used to call friend and began to push her aside, tear her down, and replace her as if she were nothing. Mal breaking his heart would definitely add to the fuel of her anger.
She’s already broken Uma’s heart, how dare Mal just saunter in and break the heart of one of her own as well.