You used the card to make purchases, now pay up, it's the right thing to do, tantamount to stealing if you do not pay the $$ back.
Yes, credit card companies make a living off of overextending credit to people who then have to pay it back with exorbitant interest rates and fees, but I am assuming that no one held the proverbial gun to your head and made you spend $$ that you did not have on this credit card.
Pay up. Karma, Man. I assume you are feeling at least a little guilty or you wouldn't have started this thread. Do the right thing...at least you'll be able to look yourself in the eye.