She is an angel in everyone's eyes, gentle as water, and kind to others. She is also a demon in the eyes of the police, killing people with ease and without resorting to any means.
She was betrayed by her closest relatives in her previous life and framed by her 'sister'. She would rather let her biological daughter suffer the humiliation of an adulterer and die than let their unrelated goddaughter be harmed in the slightest.
So after her rebirth, she swore that she would also be an angel in this life. She would be the black angel who was obviously the culprit of the crime but still made people like and forgive her.