In the Gears of War universe, millions of humans fought a massive war against an equal millions of foreign, sentient invaders called the "Locust". The war took out most of the human population along with their homes, businesses, factories, and all manner of other structures.

I recently beat the Gears of War 4 campaign, and in it the characters re-encounter the "Locust"-- where every situation is "shoot on sight", obviously!

Racism is judging someone based not on their merits as an individual, but based on pre-conceived notions. No one who plays Gears of War 4, or consumes any of the thousands of media that follow similar patterns, complains about the characters being "racist" by shooting the hostile, slimy, blood-sucking aliens, even though that behavior is indisputably so. This is an implicit agreement with the statement that not all racism is irrational or bad.

Yet, the same principle falls into play in real life, and people refuse to acknowledge it. If there is a critical mass of people from one ethnic group that commit genocide against many members of some other ethnic group, people in the victim group would be irrational to trust members of the attacking ethnicity, by default. Technically, that's racism-- but I think any reasonable, non-ideologue would call it "using your brain".

At the same time a reasonable, non-ideologue is open to changing his opinion based on new information. And perhaps that's the core difference between a "racist" and someone simply exhibiting common sense.