Racism

Many traits inflict upon the behavior of a person to be racist. In general though it's due to your education, not school-wise but how you were raised, by what kinds of people, where you were raised and how everyone around you treated each other in that place, and more. Or it could be minimal things like religion, gender, hair colors, etc. When we find a "fault", or different feature, that sets them apart from one person they believe  that they are better.
There is also a slight problem with dominative people. They feel superior and it makes them want to be at the top, thus shunning the ones that might fall lower in their perspective. Racism is mostly scene towards those with different skin colours because it is what stands out most. Another example is a very prosperous person that feels disgust upon seeing a rather poor person beg for money on the streets with ragged clothing.
Sometimes, it's not only this, but also the jealous anger that can create racist actions. If we see someone from another race, or so, doing better than we, that might cause way for racism. Or if one person is better than the rest, he'll see himself as a better creation than the others.
An important reason is that humans may feel safe to the opposing race. For example the way a gazelle avoids a tiger because it is dangerous. We might take for example terrorists, We group them from where they come from and the innocent people there. We might take them in as dangerous people. It is for cause of fear that we inch away from any possible chance we might be in danger.