So, Bob kills an innocent for an unknown reason. Naturally, Joe shoots him in the head killing him for killing an inno. Then, Suzy comes along and shoots Joe! Is Suzy to blame in this situation?
Suzy killed Joe for committing a traitorous act, no she would not be slain. Bob would be questioned about why he killed an innocent player and if the reason was valid he would not be slain, if it was not, he would.
No he's not. Though Suzy can kill Joe for killing an innocent. (Which is obviously a traitorous act) What you described is popularly known as a "RDM train" In this situation, Bob would be at fault for killing an innocent, and would be questioned before a slay is placed on him.