This is part of what makes this case so incomprehensible to most people.
Casey is not like any of the rest of us. She lives 100% in the moment. Her mind is wired in such a way that the past is the past, and the future will be dealt with only when it arrives. She is happy and smiling because she is not in jail and has her computer. She was happy when Caylee was with her, but when she is not with her she is not with her. The past has no emotional ties, the future no emotional impact. She uses lies to deflect the concerns of others, but she does not register or comprehend those concerns. At best they simply annoy her.
I doubt that Caylee's death is premeditated, simply because I don't think Casey is capable of thinking more then 5 or 10 minutes ahead. Caylee either died via a gross act of negligence, such as being locked in the trunk, or she died in a fit of pique. Casey had a huge blowout with her mother on June 15. It may have come out then that Cindy and George were considering taking action to remove Caylee from her. Caylee was Caseys primary lever of control over her parents. She may have done something brutal and final to hurt her parents. But I still doubt if it was anything she planned. She obviously drove around with the body in the trunk until the smell became unbearable. Disposal of the body would not be somethingthat she gave alot of thought to. No digging deep holes, no deep thought. Either in a swamp or pond, or more likely, in the dumpster next to where the car was found.
LE will never get a completely straight story out of her because the only leverage they have over a person normally stems from emotional attachment to the past or threats of the future. The only thing that would work on her is soemthing that deals in the immediate moment. ie waterboarding. LE had a brief breakthrough with this when they confronted her on the grounds of Universal, they physically caught her in a lie in the here and now. The only other thing that might actually get her to break regarding Caylee would be her being presented with the clearly identifiable body of the little girl. (as horrific as it sounds, a badly decomposed or unrecognizeable body probably would not work, as it is an element of the past and has no emotional ties to her, she would simply shrug it off as "that's not Caylee").
In some ways I do feel sorry for the Anthony's. Living with someone who functions like this is basically a living hell for all of those around them. As others have said, a person like this is highly skilled at playing others. Other people cannot mentally comprehend how and why these people operate, and tend to mentally twist things into a form that they understand. This always leads to unbelievable things being rationalized, and the Sociopath (lets call it what it is) gainig the upper hand.