The Good Son (1993)

A young boy stays with his aunt and uncle, and befriends his cousin who's the same age. But his cousin begins showing increasing signs of psychotic behavior. sadistic, parent child relationship, manipulation, sadism, psychopath, sociopath, deception, murder, problem child, deceit, disbelief, lying, disbelieving adult, psycho, nobody believes, uncover truth, psychopathic