He had this perception of previous social life because governments did not guarantee that citizens had their basic needs met.
We can arrive at this answer because:
When D-503 realizes that everyone has everything they need, he remembers that the previous society did not have this privilege, as the government allowed the inequality to be too great, which prevented some citizens from having access to basic things like food. This for him is a kind of mass murder, which heavily harmed the citizens.
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