Altruism is a behaviour that has been ‘naturally’ selected at the ‘social’ level,
Communities that encourage it fare better, as a whole, than communities which condone widespread indifference towards the others.
Please notice that the opposite of altruism is not ego-centrism and not even egoism but complete indifference. An egocentric or egoistic individual is one who is aware of his person and values his individuality. As such he will try to take good care of himself and never dare to behave in a completely callous manner because he fears social rejection, provided his egoism is tamed by reason.
If his egocentricity becomes unmanageable he turns into a socio-path that will be, sooner or later, expelled from the society.
If nothing out of the ordinary (extremely good or extremely bad) comes along, naturally (randomly) occurring ‘altruism’ is encouraged by some, faked by others and on the whole a ‘moderately altruistic’ behaviour becomes the modus vivendi of that particular community. Ties between the members of that group gather more and more force but don’t overwhelm the individual autonomy of the members, on one side because of the ‘fakers’ and on the other because the ‘real’ altruism involves a certain degree of respect towards the others.
If a particular social group, for whatever reasons, stops discouraging extreme egocentricity, like the one Caligula and his heirs ‘practised’ in Ancient Rome, that entire group is doomed. The largely disseminated egocentricity gives birth to indifference about the fate of the group, later to lawlessness and eventually to a state described as ‘anomie’ by a certain Durkheim – a French sociologist who discovered the link between the number of suicides taking place inside a community and the intensity of the forces that coalesce that community.
Durkheim had reached the conclusion that although the actual decision belongs to the individual, each of the members of a community is more or less ‘prone’ to consider ‘doing’ it according to the strength of the bonds that exist inside that community. (Suicide, A study in Sociology). Anomie, as described by Durkheim is “a condition or state in which there is a breakdown of social norms and guidance for the citizens of a society. Anomie occurs when society has little influence on individuals’ propensity to follow rules and norms, and individuals are, therefore, left without moral guidance. Individuals do not feel attached to the collective society.”

So, in a situation when individuals are disoriented enough as to stop valuing their own lives, what would keep a society whose members are afflicted by that malaise from falling apart?

Let’s go back in time to Caligula’s Ancient Rome or to the pre-Revolutionary France. The general atmosphere in both instances could have been very accurately described by ‘apres moi, le deluge‘ (‘a huge amount of water will be needed to cleanse after me’) – a phrase attributed either to Louis XV of France or to his mistress.
Well, we all know what followed. Ancient Rome collapsed under the attacks of the barbarians and the famous Bastille was occupied by the sans-cullotes.