"I would die before I let someone like you get hurt. I made that promise a long time ago, and I don't plan to turn my back on it now." It's what he signed up for when he joined the Alliance. Protect civilians, people who should never get involved with wars and death. Sure, the backdrop has changed, but Kaidan's morals have not. She wasn't a fighter, she didn't belong here, and Kaidan would... protect her, if she wanted it.