[Gryeyes]

First the library core wasnt disbanded unless that was retconned same with magnus teaching them all so the plot in the books made sense. And the entire "training" that a psyker receives is about the nature of the warp and how to avoid becoming tainted or possessed. It also says in horus novels that psykers were banned across the board including sorcery. All psyker powers are derived from the warp.

So we have an emperor with vast psyker powers who is aware that the warp gods want to corrupt and destroy him and his forces. Yet doesnt even brief his top leaders on what it is and how to resist it? Besides the fact magnus sure as shit knows about the warp and if he was supposed to have trained the librarians naturally so would they. Ignorance of the threat of the warp is how beings get corrupted and possessed. Its why all pskyers are controlled and trained or killed. All psykers are rare so by default so are marine psykers. But ANYONE with psyker powers is open to possession and the warp. Its one of the few ways daemons can even enter the material world.