MikeH

Answer: At the end of the day, he's still her father, and she's a young child. It would be difficult for her to give the FBI the smoking gun.

Answer: No. She didn't get along with her family, but she was a basically kind girl who still looked out for her father.

Brian Katcher

Answer: Kids generally like to see the best in the people closest to them, and while Harry and Zinnia were mean and neglectful, Matilda still had a little bit of love for them (and/or sympathy for how stupid they actually were) and didn't want to see them go to jail. Part of it might have also been because she knew they wouldn't survive very long in prison. And besides, she got what she wanted. To be with Miss Honey.

Answer: The "real world" answer is that it's a movie, and without Trunchbull, there's no villain at the school for Matilda to battle. Within the context of the film, everyone, including the staff, is afraid of her and fear if they speak up, she'll retaliate.

Answer: They would have been simply too afraid of the Trunchbull to report her.

Chosen answer: Yes she did. Whatever way she killed him she made it look like a suicide so that's what the police ruled it as. No reason to arrest her if they ruled it a suicide.