She wrote Jess as a total A-hole because she wanted to adhere to the damaged bad boy trope because she knows women respond to those kinds of characters. They like seeing the girl take sympathy on the damaged bad boy and want to fix them OR see the bad boy through knowing and loving said girl changing her and helping her to see her true inner self. It's such a trite and predictable trope that sucks way too many impressionable young girls in so that they look for the damaged bad boys IRL to "fix". That's just so dangerous in my eyes.