My nation is young, and built by the work of explorers; Well-traveled Australian weighing in.
Let's start here: Species is human, race is breed.
When it comes to race, I believe the anomaly isn't in killing those who look different, it's in not killing those who look different. From a reactionary standpoint, somewhere around two hundred years ago, progressivism became the 'left' and suddenly, very very suddenly, racial sensitivity became an issue. Imagine a pack of wild Border-Collies, and a pack of wild Red Heelers. These two packs will always, unconditionally, attack and kill each other over territory. If, say, one of those packs decided that "Hey, we're all 'dogs', lets just hang out and make a bigger pack", they'd likely be obliterated by the opposing pack. This is true with prides of lions, packs of wolves and troops of apes. Why? Strength and aggression win in nature every single time. What can we glean from this example? Human groups weren't designed to intermingle.
OK, give yourself a second; I just said something nasty and you need time to deal with it.
The implication of what I just said is that all humans aren't equal. Gasp. Reel. Faint, if you have to. OK, let's take some examples from ancient history:
Here's a race of people who rose up out of the dirt (for argument's sake) in northern lands. To succeed in these lands, man must gather in groups and settle in one location for long winters and short summers. Conquerors of this location must spread their land-holdings to larger frontiers. The land is kind and fertile, and can be tilled and farmed without exhaustion, removing the need to relocate. The state must care for it's people on an individual level or risk rebellion. Europe.
Here's a race of people who rose up out of the dirt (for argument's sake) in southern lands. To succeed in these lands, man must be individually strong to constantly lead other groups to more profitable lands. Conquerors of this location must meet and kill competing leaders with their power, which, in victory, attracts more power. Although the land is too harsh for long-term farming, constant relocation guarantees sustainable nutrition for small groups of (<1000) people. The state must disregard it's people for fear of appearing weak or risk revolt. South America (Aztec, Inca), Middle East (Iraq, Persia), Southern Africa.
Here's a race of people who rose up out of the dirt (for argument's sake) in equatorial lands. To succeed in these lands, man must be careful about exhausting the fertile lands, and engages in ritual human sacrifice, blood sport, marital restriction, and worship of brutal Gods to keep population regulated and controlled. Conquerors of this location do not contest for lands, but must keep their subjects fearful of divine retribution to prevent dissidence and rebellion. The state must use fear to control it's people or risk cultural abandonment. Northern Africa, Egypt, India.
Here's a race of people who rose up out of the dirt (for argument's sake) in equatorial lands. To succeed in these lands, man must be careful about exhausting the fertile lands, and engages in ritualistic suicide and oppressive border control. Conquerors of this location must use the male sense of honor to motivate men to death, as well as war, as a means of population control and maintaining state control of the people. The state must use pride and honor to align it's people, or risk revolt. China, Japan, Korea, also India.
I do not believe that race is a social construct. If it is a construct definable outside of biology, it must be geographical. I mean, it must be, right? What the fuck else can explain the differences between races of humans?