“Sometimes I feel like it needs to be pointed out that the reason Gia and Cobb were able to frame Logan for Carrie’s Murder wasn’t because he was a “bad boy,” it was because they knew that despite Carrie cheating on him, despite their break up and despite the fact that Carrie had yelled at him earlier that very day, if Carrie asked Logan for help he would do the right thing.”
I didn't really buy Carrie Bishop's transformation into Bonnie Deville at first either. I just couldn't picture the Carrie Bishop we knew from Neptune as Bonnie Deville. I assume though that Carrie transformed into Bonnie after Susan's death on the boat. We know Susan was her best friend and it must have been a traumatic experience. My guess then is that Susan's death is the reason Carrie changed. Maybe she needed the alter ego to deal and express herself and she didn't feel like Carrie anymore.