I did NOT like what they did to Erin's character in season 4.
If Erin loved Vlad SO much and accepted him for what he was, I simply can't believe that she would react to becoming a vampire like that. It really doesn't make sense to me, especially because she was DYING. He didn't bite her on a whim or because he was afraid that she was going to break up with him...she was dying. If he hadn't bitten her, she wouldn't be able to hate him...because she would be dead!
Now I could see Erin getting lost in the power and losing control like Vlad did when he looked in the mirror, Vlad chasing after her to stop her from biting people and then Erin breaking down and sobbing...but I can't see Erin hating Vlad for changing her and becoming evil.