I mainly believe that most people are a product of their circumstances and how they have decided to cope with their experiences. But imo, a good person regularly does things that benefit others without any benefit to themselves. Especially if they're not obligated to do it. Good people are kind (but not a pushover) to everyone, even if someone is unkind to them.

On the other end of the spectrum, bad people do things that hurt others even if it is of no benefit to themselves. They are unkind even if it takes nothing off of them to be kind.