If humans are innately ”good”, why does anarchy and barbarism usually crops up when government breaks down?

I have heard it said often, here as well, that humans/mankind is innately or inherently good. Time after time when government breaks down because of war, natural disaster, etc. it disintegrates into ”big fish eat little fish”, if not stemmed by order being restored; it would eventually end up like many apocalyptic movies where the gang with the most guns rule the roost. Why does it happen if people are innately good? Is looting and such as during Katrina, or savagery as in Somalia, Darfur, Rwanda, etc. isolated examples of systemic mental illness and unnatural behavior? When temporary anarchy breaks out such as the Rodney King riots, it was ”good people” being overtaken by some force that made them a menacing mob, instead of the helpful, peaceful, benign people they are supposed to innately be?