How so? What more would it have taken?
I think it's plausible that the little girl's death was accidental, probably because Casey gave her Benedryl or something similar to get her to go to sleep so she could go out and party, and she gave her too much. And she and her parents then tried to cover it up.
I seem to recall that the jury did not have the option of finding her guilty of a lesser charge, such as manslaughter.
It had to be guilty of first-degree murder or not guilty.