Pretty Poison (1968)

Easygoing but psychotic Dennis is released from jail, where he has served a sentence for his complicity in a suspicious death. Wandering through an Ozzie & Harriet-type small town, Dennis makes friends with a seemingly normal high schooler, Sue Ann. He lies to her about his imaginary career as a secret agent, and she is thrilled to the point of joining him in his further adventures. Their developing relationship becomes a lethal combination as Sue Ann uses Dennis's undying devotion and his bragging about being a secret agent for her own evil ends.