'Completely' is a meaningless term. Animals behave selfishly in almost every choice (even in altruism) as part of the survival instinct. There is a question of degree and there is a distribution curve that hasn't been fully explored. For some people, some desperate or casual situations lead to barbaric (lack of a better term) behavior, when possible, for some portion of the population. Even with a police force, the curve exists and we experience the effects.