Frau may have intended on staying quiet, but he couldn't anymore. He needed to make something very clear, very quickly.

"He's not going to die. I won't let him."

And he wouldn't. Teito would not die. Frau would do anything he had to do in order to make sure that was a reality. Whatever it took, it didn't matter. If Teito left him alone for good, Frau wasn't sure her could handle it. Even more importantly, Teito's path had just begun, and he had far too many important things to do in his life for it to end here.