While watching the last episode "American Gothic", I have been thinking that at the end of the episode when Katherine gave Elijah the cure and trusting him with it, Katherine does not give up. The cure was her bargaining chip with Klaus. After she gave it to him, she walked away. Meanwhile, Elena was last seen knocked out by Katherine after her conversation with Elijah. I believe that Katherine switched clothes again with Elena and called Damon and Stefan to meet her. When she threatened them to give up getting the cure for her, I believe that it was really Katherine because she gave the cure to Elijah to give to Klaus and she does not want the brothers to continue searching for it, etc. I also think that it is a possibility that she dyed her hair like Elena's and will be at the Prom as well. Even though it seems that Elena is really gone off the rails with the no humanity switch, I cannot see her killing people nor threatening to do so. That is more Katherine's MO. Katherine is a master at imitating Elena and since she now knows that Elena's switch is off, she is going to make the best of a situation but for her own benefit. What do you think?