Rude obnoxious kids, product of parenting, or they are just born that way?

I am not sure if anyone else noticed, but I notice that some kids are way more nice and respectful even more than kids that are years older, and some kids are just rude, obnoxious and seem clueless that the world doesn’t revolve around them. Why the big difference? Are some kids just born that way or do their rudeness come from how their parents trained them, or severe lack thereof?