That is a mystery. My paternal grandparents were really nice people, but my father is awful. My stepfather was a violent criminal, but his parents were two of the nicest people I ever knew. So somehow, these four nice people produced two sons who are the worst people you can imagine (my mom was attracted to that type) Maybe parenting doesn't produce reliable results?

Maybe your experience of having mean parents taught you sympathy because you remember being sad or anxious so you recognize those feelings in others. I often feel that people who had nice parents lack empathy and are judgemental because they don't understand how things like abuse or financial insecurity affect other people. I also think I understand other people's feelings well, because I was an anxious kid and used to watch people carefully. So maybe you were affected the same way.