I would leave. This kid is seriously disturbed. There is probably something deeply flawed in the dynamics between the kid and his parent(s.) What about the father? If you stay, you are at risk of getting somehow blamed for continuing mischief this kid is going to get into. (And "mischief" understates what I can imagine this kid doing.)

This child needs to be in the care of someone trained to work with mentally/emotionally disturbed children. If I were you, I wouldn't take that responsibility. Five years old is too old to behave like that. Something is really wrong in that home. I'ld get out of there.