Almost my exact thought. Big thanks pal. Accessory?
Probably not. He has to have had the same purpose, or contribute. Keeping quiet is not contribution. There is no general (legal) duty to the public, to rat out a suspicion.
Not likely. Unless the killer had given the room mate good reason to believe that he was serious and believable about what he intended to do I doubt that he will be held culpable.
A great many people make idle comments and/or threats about committing a crime or exacting revenge for something or other, but only a very few of those threats are serious and followed through. If all such threats were taken seriously and investigated by LE they wouldn't have very much time to do the more serious work.