I think this behavior was probably common when it came to casey's parenting skills. She just hid her true feelings around her family and friends but maybe was more obvious about her dislike for the child when no one she knew was around.
I don't think she beat Caylee to death. I don't think anything in particular set casey off that day. I think she planned the destruction of her child for months, hence the computer searches for missing kids, chloroform, breaking necks, etc., in March, 2008. casey decided to finally do it after a cumulation of events, such as her boyfriend not welcoming Caylee to spend the night and saying he only wanted boys, not girls, her mother becoming more suspicious of casey's employment status and refusing to babysit as much on nights, telling casey insetad thnat she needs to get a job and watch her own child, casey's friends getting to party whenever they wanted to and go on cool trips while casey had to sit home, Caylee getting old enough to articulate where she went with mom and finally, casey being caught stealing from family, causing a huge fight between her and CA on Father's Day. All of this made the moment "right" for casey to go through with a plan she had in the works for months.
I think in casey's mind, her mom would not come down on her so hard and casey would not have to steal so much if Caylee were not around because then, there would be no,"You need to take care of your own daughter, your "mistake". You are bad mother. You need to be repsonible and get a job." casey had to come up with material goods to prove she was working and get her mom of her back. She also linked these admonisions with Caylee because if Caylee was not around, casey thought CA would have no reason to pester casey for being "irresponsible" and not working. Also, she might be given a lot more freedom and allowed to sit around not working if she was seen as the grieving mom of a missing kid.
No, I don't think Caylee's murder was a result of a mom snapping and suddenly killing her kid. casey is a cold, sinister, pre-meditated murderer of her own, beautiful little girl. The one who was taking her place as the princess of the family and the one who, by her very existence, was relegating casey to Cinderella status in her very own home. Killing Caylee would thus allow the rest of the Cinderella story to occur - the evil stepsisters and stepmother would be rendered irrelevant and casey would be free in a lovely, fairy tale world and finally get her prince.