Originally Posted by FoldnDark

Probably something to do with selfishness propagating the species and has always done so. Imagine a tribe of nomads fighting over a good'ol fishin hole or something, the war would rage until one side was vanquished or gave up and moved on. So, if one tribe was down to say 30 people, but found some way to kill the 100 remaining members of the other tribe, would they hesitate? That winning minority would go on to pass their superior (more strategically intelligent) genes.

In modern times we continue to value those we care about more than the masses of unknown "tribes." In a sense we are in a war with everyone else. Save your wife/mother and **** the rest. As pointed out the movie is an extreme example which is highly improbable, but nonetheless if those circumstances played out that way a human might make that choice. A more relevant and likely scenario would be the old Cold War where we came pretty close to destroying the world to protect one tribe.