What the hell was that ending?

Hmm...kid murders his innocent parents.
His dying mother uses her last words to warn him that he shouldn't let her out - which suggests that she's doing it out of concern for him.
He himself shows doubts about whether he should release this unknown creature, but decides to do it anyway, BY HIMSELF, instead of calling the cops.

Then him and Miss Devine manage to somehow trap the monster in the cage.
The monster IS actually his sister. Two normal humans manage to somehow give birth to a monster with superspeed, superstrength that literally pulverizes humans but is not enough to break through a thin wall.

The monster warns that this cage won't keep her down here. Movie ends.

Ummm.......what?
He's going to keep her there and not reveal anything to the authorities?
How is he going to explain all the dead bodies to the police?
Why does he not seem bothered by the fact he killed his well-meaning parents?
WTF is this ending?