Here is my opinion...
I believe that Casey killed Caylee while in a fit of rage, after a fight with her mother, Cindy. An act of revenge, to 'get back at' Cindy.
During the initial questioning with the detectives, Sergeant John Allen
specifically asked her if it was an accident, he even mentioned drowning. Rather than go for this out, Casey decided to keep up the implausible, ridiculous lie about a kidnapping nanny that didn't exist.
At that point in time, Casey could not claim an accident, because to do so would mean she would have to lead them to Caylee's body.
By 'admitting' to an accident, she could have avoided extensive jail time (as it was she only ended up doing 3 years), and avoided death penalty charges. But, she would have had to let them know where Caylee's body was, so they could recover it.
Only one problem with allowing law enforcement to see Caylee's body, though.... there were three strips of duct tape over Caylee's face.
~And, if Caylee's body HAD been found earlier, something tells me they would have found her throat had been slit, with that steak knife that Cindy said she found and washed when they recovered the car. As horrific as the duct tape and chloroform were, I think that was the least of what Casey did to Caylee. I really think the duct tape was used so that nobody could hear Caylee scream.
Casey's rage would not have been quenched by merely putting duct tape on that defenseless two year old's mouth. I believe that Casey killed Caylee in a burst of extreme violence.
There was no way Casey would 'cop' to an accident. There is no way she would lead them to that body.