An interesting thought, but I think this is one of those social science hypotheses that you're just not gonna be able to prove.

Your experimental measures are not strong enough to answer this question. And if we find some way of measuring society-wide guilt other than your subjective selection and interpretation of lines from ancient plays, and if we suppose hypothetically that this still backs up the conclusions of your incredibly subjective survey, there can very easily be a common cause (technological progress, increasing "liberal drift"), reverse causation (maybe cultural success allows for more guilt), or just random chance at work since the sample size is small.