Carlyle actually told Edward to "remember who you are" when Edward was sucking James's venom from Bella's wrist.  Edward's adoptive father was referring to the fact that Edward, like the rest of the Cullens, was a vegetarian who had made the decision not to kill human beings.  Carlyle also meant to remind Edward that he was in love with Bella, and would always have to deal with the very present temptation to kill her due to the scent of her blood.  Carlyle knew that Edward could easily lose control and kill Bella in this situation, as he was sucking her blood with increasing vigor, but his reminder brought Edward to his senses and Bella was saved.