THEORY:
Cindy and KC argued, it may have escalated into a physical fight. Cindy called KC an unfit mother; KC stomped out of the house, Caylee in tow. That was the evening of the 15th, not the morning of the 16th. (I think George may have lied about seeing them leave that morning, probably to protect his wife/family--likely at the time he didn't think the fight was relevant to Caylee being MIA and didn't see the the need to air that dirty laundry.)
KC and Caylee hid out the night of the 15th at TL's apartment. The next day (June 16th) TL tells KC he is just not comfortable with Caylee spending time at his place as it is not an appropriate environment for a shall child, especially a girl child. KC calls Cindy to see if she can drop Caylee off, but Cindy is unavailable to babysit.
KC is angry. If she can't find a sitter she will have to cancel her plans with TL. This is most inconvenient for KC, not just for this evening but she is likely to face many more evenings when she has no sitter. On top of all this, now Caylee is getting tired and fussy.
KC decides to do away with Caylee 1) her own personal freedom, 2) to get "even" with Cindy, 3) TL has finally admitted he doesn't think Caylee should be subjected to the lifestyle of him and his roommates. But, it is a lifestyle KC wants for herself.
This theory does not explain why KC killed her daughter rather than give her up for adoption but people often do things in the heat of the moment that defy explanation. Maybe if KC had thought about her situation to any degree she would have realized her other options, but she had finally met a guy she was really into and she was just sick and tired of motherhood standing in the way of her life. KC did not want to let TL get away, so she sent Caylee away forever.