The comments from Joshua Weintraub came Monday in the first day of trial for Geralyn Graham, 66, the Miami Herald reported.
Graham was the state-appointed foster mother of Rilya Wilson when the girl disappeared in 2001. The girl's body has never been found.
Graham is charged with first-degree murder, kidnapping and aggravated child abuse.
Weintraub told jurors Graham hated the girl, restraining her with "flex cuffs" inside a dog crate.
"She thought, after eight months, the child had demons and was evil," the prosecutor said in his opening remarks.
Defense attorney Scott Sakin put the blame on Florida's child welfare agency, which had placed Rilya in Graham's care.
Without Rilya's body, prosecutors can't even prove that she's dead, Sakin said.
Rilya's disappearance created a tsunami of criticism for the child protection agency. The girl's disappearance was not noticed until April 2002 because her case worker created false reports that indicated he had made regular home visits.
Publicity about the case led to a series of reforms at the agency and legislative changes.