Elena let herself lay back down when Stefan agreed. Her demeanor calmed quickly once she knew that he wouldn't be running out to immediately try to kill Katherine. "I want her gone, too," she told him. "So much. All of the trouble she has caused... but if I were to lose you I couldn't bear it." She may not have needed to explain, but she felt like she should. She wasn't defending Katherine, not in the slightest. She was just protecting the man she loved.