Our Own Worst Enemy

I read a quote the other day that said "There is no witness so dreadful, no accuser so terrible as the conscience that dwells in the heart of every man." As I pondered on it, I asked myself: how reliable is our conscience? The little voice inside your head saying "this is wrong". How effective is it? When tempted with something we want, yet something we know we shouldn't do, why do we do it anyway? If our conscience is screaming at us to stop, why do we proceed? I think it's because the want and desire often overpowers the morals we have. We'd like to think we'd do the right thing, but when actually faced with the decision, we're hesitant. How bad does the consequence of our actions have to be before we stop ourselves? I'm not saying everybody always makes the bad choice, but I think it's safe to say that if we want something bad enough, we'll do what we need to in order to get it. So again I ask, how reliable is our conscience?