Why do people insult my late absent mother?

My family called her cold hearted and selfish woman that didn’t give a rats ass about me for a decade. My late dad even called my mom a bitch right in front of me and didn’t care. He said I’m old enough and he can say those words around me since I’m an adult. I had just turned 18. My best friend called my mother a bitch for abandoning my dad and why am I grieving over a woman that didn’t want me. She didn’t deserve empathy.