24 Sep 2012
This is what Anghel says just before his epic(ly short) battle with Ryouta...and I had a thought on what the "Pact of Old" refers to, contributing to my theory of Anghel and Nageki knowing each other back in the medical center:
After Nageki decides to take his life, he makes Anghel promise to ensure that the Charon Virus truly is gone forever, and that it never returns to kill another human being ever again. This promise, I believe, is the "Pact of Old."
As such, Anghel, recognizing Ryouta as the new carrier of the virus, and the new Weaver of the Song of Death, decides that he must either be destroyed or sealed away within the medical center, thus the attack.