Confused about Bayek's early motivation in ac:o [maybe spoilers]

Don't know if anyone can help me figure out my situation. I've just started origins and really an enjoying the game play. I'm about 30 minutes in but I have no idea why I'm killing the masked ones. Don't know if I missed some crucial detail or if I'm supposed to be in the dark about it, but I just don't really get what's going on, so if anyone can help me out that'd be great