Just to start this out I would like to say that I highly doubt any of this is correct so this is more like a wish then a theory. Having recently grown very interested in Medicant Bias I read alot of theories which were actually considered to be mostly credible. From all of these theories I have gathered that Medicant Bias actually played a huge role in Halo3 by holding part of installation 4 (I think thats the right number.) and possibly influencing where the half of the Pillar of Autumn ,that Master Chief was on, ended up. It is widely assumed that Medicant Bias died when installation 4 fired which I entirely agree with but after what Medicant Bias did to save the chief I am not sure if his story will just be tossed aside.
Anyway what I am leading up to is that maybe Offensive Bias will be in Halo 4, possibly as an enemy since the only reason he was made was to destroy Medicant Bias and since Medicant Bias helped Master Chief so much maybe that would make Offensive Bias consider Master Chief an enemy. If this is true it might account for the ancient "evil" since Offensive Bias' only goal was to completely destroy and dismember another AI which could be considered evil. I am just thinking aloud and with no evidence to back up my claims. Ok feel free to tear this theory apart now.