We split up people. We all do it. We categorize. We create these little "us and them" categories. Some of this is just a way to identify yourself, to figure out who you are, and to identify others, to figure out how you are different from others. We do it all the time. We do it in an unconscious, blink of the eye manner.
Some of this is just biology. Some of it's probably a survival thing. "This person is like me, I'm probably safe." Or "This person is not like me, I might not be safe."
There are an infinity of characteristics we use to make these decisions. This can be useful, but can also lead to all kinds of problems. This "us and them" thing allows us to see each other as categories, or as a collection of characteristics, and not as complete human beings.
Really, most of the categories are arbitrary, cosmetic.
I think there are really only two questions (The Mother of All Categories!) we need to ask when we consider another human being and they probably take time, observation over time, are difficult to discern, and probably can't make it in a snap judgement kind of way:
1. Do they intentionally try to alleviate the suffering of others? These are the best amongst us.
2. Do they intentionally try to increase the suffering of others? These are the least amongst us.