>> "Granted, even this information gets tainted: narcissist parents, for example, might come across as their parents being nice, but them not being nice to their parents."

You can tell a lot about someone by how they treat someone they don't like. Some people get downright cruel with people they don't like, and it's a huge red flag because some day they may find reason to not like you. You're just setting yourself up to become a target once you find yourself on their bad side.

You sometimes see this when someone's hating on a public figure. They insult the person's weight, appearance, and speculate about their sexuality. I am never surprised when these people turn out to be abusers and harassers.