Romeo uses irony to persuade the apothecary.
Romeo points out the apothecary is "full of wretchedness," meaning that he is clearly poor and near starving. He says that if you are poor, starving, and near death anyway, you might as well break the law and sell me the poison. He says that the law is not in place to make the apothecary rich, so he might as well take the money and survive.
Ironically, by selling a deadly poison the apothecary is making money to live.
