"Mitigating" means that, um, your father had a reason to commit murder, such as Danny Chaidez tried to rape you, which is what you told him, even though we both know that's not true.
"Atenuante" significa que si tu padre tenía motivo para cometer el asesinato, como que Danny Sanchez intentara violarte, que es lo que le contaste, aunque ambas sabemos que no es verdad.