Earlier generations were punished because they rejected the Divine message and were involved in sins. Allah (SWT) changes His grace to punishment when people change their belief to infidelity. The infidels are regarded as the worst creatures and their habit of breaking agreements is high-lighted. Muslims are ordered to throw back their agreements if they fear betrayal and to punish them in such a manner that their later generations too should be cautious remembering the consequence of their forefathers.