Given what is publicly known, I don't see this as the slam dunk many here do. What we have is YingYing willingly getting into a car that was owned by the accused. That hardly constitutes kidnapping. Then, it is not conclusive that he was the one driving the car, the pictures we have do not show him clearly.
When he was taped talking about picking out other victims, I believe that the FBI felt they needed to act before there was another possible victim but still may not have had more evidence than she got in a car owned by him. Given what is publicly known, the evidence seems weak for even the kidnapping charge. Even despite what he said, he doesn't have to defend his comments. The Feds have to prove that he took her against her will, and what I see is someone who willingly got in a car. Just imagine showing that video in court. "Does Yingying appear forced into the car?", defense attorney X asks of the lead detective. The reply can only be, "No".
Unless there is a body or a WHOLE lot more evidence, murder is a huge stretch.