I don't think the virus could have come from X. The potential to think for one's self completely, and thus the ability to consider harming humans came from X. I'm thinking that the virus came from Zero and alters the thought processes of a reploid so that the consideration of harming humans becomes a certainty.
To this day I'm STILL a bit bewildered by Capcom's claim that the Suffering Circuit/Angst Circuit/Artificial Conscience is the cause of Maverick behavior, tell you the truth.
I like Zero's DoS explanation better: Short circuits or program errors, plus a dash of REASONED resistance (aka: "Hey, humans treat me like shit, ergo, I now dislike humans.") I think he's spot on that "the very things that give us our advanced processing capacity could also be our greatest weakness."
Any sentient being has the ability to make choices- some good, some bad. Personally, I far prefer it if Mavericks become so "of their own accord", which both Lumine and retcon!genius!Sigma place a great emphasis on.
The Suffering Circuit, however, could be malfunctioning in other Reploids because it wasn't fully analyzed by Cain and so incorrectly installed. It could also be that a combination of Zero's original Virus provides the spark that drives normal Reploids over the edge, either annihilating or in some way mucking up the code of the Suffering Circuit to cause a malfunction.
BE NICE IF WE HAD AN MHX 2 TO HELP FIGURE THIS OUT. ;p