When you watch someone being mean to someone else, you feel bad. When you see someone being kind to someone, you feel good.
Why does this happen even when you are not part of the behaviour, recipient or even know the parties involved?
This happens because we are all hardwired to do good and serve others. And that is because we are all interconnected at the root. What affects one affects all.
Empathy is born out of interconnectedness.
Therefore, when either you or someone else is doing good it feels good even to watch.
At the same time, we are hardwired to be hurt by bad behaviour towards us or someone else. Therefore, when either you or someone else is displaying bad behaviour we feel bad.
This happens to us even if the incidents are happening in a movie!