A deep insight.
The most important thing Richard knows is that while conscience allows us to understand ordinary crimes, it actually blinds us before the most extraordinary ones. The idea that conscience blinds us, making us less able to oppose evil's most brazen forms, is deeply disturbing, for conscience is the sine qua non of civil society. Conscience is supposed to be the faculty that helps us become aware of our effects on others and our motives towards them, notably our baser motives. In Elizabethan English, "conscience" is an equivocal word that can mean either that faculty that allows us to feel guilt or "awareness," as in "consciousness." When Hamlet says, "Conscience does make cowards of us all," he means consciousness, by making us aware of the possibility of death, makes us cowardly.
This is a problem far older than Shakepeare, having shown up in the Bible. Israel's first King, Saul, is eventually deposed by G-d because he failed to destroy Amalek completely. King David, Saul's successor, even had the same problem - though in the end he acted to obey G-d's command. Both were afraid of what we would call "public opinion," as are so many modern politicians. One would hope that a man who claims such moral clarity as Bush would act deceisively to crush evil on more than a selective basis.