So Cortana is a villan as of halo 5, and has lots of threads here on waypoint. I have an explanation that I have not heard before, and thought I’d share.
These are just thoughts that came into my head, no research or major thinking involved, so feel free to prove me wrong.
Enclosed in the next spoiler is a red vs blue spoiler and the next guess. You have been warned.
So in RvB they tortured alpha and he split, and well I know these are different universes, maybe they have similar A.I., and Cortana going rampant and splitting her evil emotions off isn’t too far of a stretch.