> I don't understand this. Shouldn't the behavior be constant?

Spoken like a programmer. Adults aren't even consistent. Ever seen a woman going completely apeshit at a male for some reason (sometime justified, sometimes not)?

Biological organisms have significant variations. Sometimes you get an angel; sometimes you get a devil; sometimes both.

In the past, resources were sufficiently scarce that children (and adults) who weren't capable of fitting into society were killed or allowed to die.