The disturbed Staten Island teenager charged with decapitating his mom’s boyfriend will seek an insanity defense, his lawyer revealed Thursday — saying the accused killer is schizophrenic and was off his meds at the time of the gruesome crime. Damien Hurstel, 19, only spiraled out of control after he came of age, meaning he was no longer a minor under his parents’ care and his mother was barred from forcing him to take his medication, defense attorney Mark Fonte argued. “When he’s medicated he