Why does my ex hate me so much even though she did me wrong

Like she still got a grudge that after the break up I made a fake account and told her new bf that she’s a S1ut etc even tho I felt bad doing that I re added her bf again and told him she’s not like that and that I was lying . Even though she basically in a way cheated on me as she was already getting with this new guy before we fully broke it of and once when I questioned her about him she told me not to worry and said “his like a brother “ so I don’t understand why does she act like I’m the one did her wrong