It's not only bad things you have done but those
acts of goodness you missed. A man has a
potentiality to do good things; to progress in the
paths of virtue and
self-improvement. It is clear that no one is able to avoid lapses and that's something where bad conscience derives from. Furthermore, you don't only need to make lapses but your
unused potentiality makes you feel bad.
See also Original Sin.
For a layman, to say it in other words, original sin is bad conscience. I think ximenez had the same idea in eir wu but approached the question from a different angle and therefore the result is different.
See Guilt and Hedonism - to live up your desires is never the solution for bad conscience. You will always have time to think and when you think... that's when you dive into bad feeling without a snorkel. Those who have drunk know that hangover has also a psychological dimension.