Sun-Mei is saved from Zombie-dom, but has fatal injuries from her fight to survive. She knows that she is the key to saving humanity, but only if she dies and the team takes her brain. She tells them how to poison her in order to extract her brain to make more of the vaccine and OMG this is really devastating. Sun-Mei!
Estes is furious when he finds Sun-Mei healed, and yet dead. And sees that her brain is missing. He knows Warren, George and company are keeping the vaccine from him and he orders these "traitors" all killed. OMG - what's next?!