When Edward was about to kill James, why did Carlisle stop him and tell him to remember who he was?The Cullen family killed him anyway. Why couldn't Edward be the one to do it?
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.
Carlisle had to make Edward remember that he cannot turn into a blood thirsty creature and especially when Bella was so close by. If he became blood thirsty and violent enough to James he won't be able to control himself towards Bella and hurt her instead.
Carlisle was reminding Edward to not get so caught up in the blood lust of killing James especially since Bella was around. If Edward was to succumb to such a state of mind then it would've been more dangerous to Bella than helpful especially since he had just about finished sucking the venom out of Bella.
He actually says 'remember who you are'. Edward's fury and anger would have over taken him and he would of killed James and maybe not have been able to stop that anger from ruling him. He might have become a monster, carlisle didn't want that to happen so he stopped him and reminded hi that he wasn't a monster and that bella needed him.