Again I agree with you, whatever happened to Caylee was no accident. I think Casey's jealousy, her desire to party rather than be a mom, Cindy's (I'm assuming) demands that Casey straighten her act out and take care of her daughter - it all led to Casey resenting Caylee. I doubt Casey ever truly had any real maternal feelings for Caylee to start with. She just used her as a tool to get her way with Cindy and George.
The odds of any child suffering a fatal accident are relatively low. Factor in a mother who resents the child and considers the child to be a burden --the odds that it was an accident go even lower. When the mother doesn't report what happened, hides the body, lies about the child's whereabouts, and carries on with her life without a care in the world.......no way was it an accident.