Gladius Legis
Legend
It's been a long time since I've actually played it, but I do not recall it being portrayed in a way that would make sense for your character to know who it is before you ever got there. That guy who wasn't a "real baddie" was still collecting the same exact thing you were after at that stage in the game, and walking around with demons and devils and killing people and raiding villages to get those.There is no choice for your character to figure out that the guy you are pursuing isn't the real baddie and act accordingly, even though as a player you figured it out ages ago
It was rather clumsily telegraphed, to be sure, but I've seen far worse examples of what you're describing. It also doesn't make the whole campaign bad as you suggest.