Demetri was the first that had to go down. Without him, Aro was sort of blind. He wouldn't know where she, Afton, or anybody was. But killing Demetri was as hard as killing Aro, with the exception that Demetri didn't have Jane and Alec to protect him. Thankfully.
"Of course he will. Everyone around Caius is a mere tool except his wife. But we could use him. His protection is what we need until we see the right time to leave." She turned her face to look at him. "He will protect me because he needs me. He knows without me the bonds of the vampires who have joined him willingly will eventually break if something goes wrong. He needs me to make them strong."