Well it's pretty well documented, for instance in the case of natural disasters, that our layer of civilisation peals off very quickly when the control mechanisms are disabled, and when those control mechanisms are part of the problem ; Rwanda, Auswitch, Pol pot, Stalin. Etc. etc.
I suppose the point is that people can become divorced from their sense of belonging or lose their moral compass as a result of negative influence or experience. 'Good' and 'evil' are ideals, and they are nice to have, but acting either way is still reward seeking behavior. We are driven by much baser principles than we care to admit.