Why can't people just be good?

I often find myself thinking what leads people to bad?

Why do people become evil? Surely not all of them can be mentally unstable.

Okay, not just evil what about just generally bad? What about bad manners? Or men that treat women improperly or vice versa.

Do people not stop to think about how they themselves would like to be treated?

Treat others how you would like to be treated yourself. Want for your brother what you want for yourself. Isn't it that simple?