Going along with your conclusion, the question remains, why? What would motivate her to (presumably) knife her little girl to death? What was to be gained?
Casey Anthony is 22 years old. She likes to drink and party and have sex with random men she meets.
She never wanted to have a kid in the first place, and her mother had to talk her into not giving the little girl up for adoption.
Kids, especially two year olds, cry and complain a lot and have to be looked after constantly. That really cuts into your party time. Plus, the men you pick up in bars are not too jazzed about dating women with kids and aren't too understanding about calling you back when you leave the party early to pick up your kid.
Caylee was an enormous pain in the butt for Casey and was totally ruining her desired lifestyle. An enraged, hungover and resentful "mom", confronted at two in the morning by a screaming toddler could likely form the conclusion in her mind that if she just shut this toddler up for good and all, she'd have her life back.
sociopathic self-centeredness
the child was “holding her back” from a fine lifestyle in the “Champagne Lounge” rooms of the VIP clubs and bars.
There are people like her, both male and female, that step beyond the bounds of civilization.
Motivation, in the sense of a something that could be understood by you and I in a rational and moral manner, does not surface in many of these cases.