the vast majority of them don't realize that what they're doing is not good. If that were the case, wouldn't they be torn apart by a guilty conscience? How does a person live, realizing that their whole life revolves around bad deeds?

Very easy to answer!

These people have learnt that their lives are easier if they do NOT weigh down their conscience. So they suppress their thoughts and talk themselves out of their actions. It's not witchcraft and it's not an art, we can all do it and we all do it.