No moral safeguard

Connections List
 * Atheist Inconsistencies
 * Value System

Atheism has no ultimate motivation to be a good person. It is useful for the society as a whole to follow common morals but it is only useful for an individual to follow them insofar as the risk of getting caught is higher than potential benefits.

Also, no reason not to cull sick, old, etc. It's the ultimate form of sacrifice (assuming you expect people to follow the laws for the good of the people).