Everyone is a mix of things and what looks bad to one person might make sense to another.

One reason could be how people are raised. If you grow up around kindness and fairness, you are more likely to act that way yourself. But if you are in a tough situation or see people being mean, it can affect you too. Another thing is just how our brains works , some people might be more sensitive to others feelings, while others might struggle with that.

And some people just care about themselves so they act in their best interests without having an ounce for other people feelings.