Revealing mistake: In the scene with the games console the controller isn't on and it is either not attached to a PS3 or the PS3 is off. You can tell because if the controller was on there would be a solid red light and even if it was just charging there would be a blinking red light.
Revealing mistake: When Amy kills Desi, we can see his neck move, meaning he's breathing.
Answer: To my knowledge, the purpose of hitting herself was two-fold. First of all, a facial injury would support the idea that she was abused or in some sort of bad situation. And secondly, to help mildly alter her appearance, so she could blend in more. (Especially considering people were looking for her).
TedStixon