The Null Primarch Wasn't Destroyed — He Was Hidden for THIS

In The First Heretic, five Word Bearer captains pass through the Emperor's gene-vault as insubstantial phantoms — ghosts of a future not yet happened, shown by a daemon the very moment the Primarchs were born. They move through twenty pods. At nineteen of them, they don't pause. At the eleventh, Argel Tal stops and looks through the glass. Canon records his observation: the Eleventh Primarch was "still innocent, still pure." His companion Xaphen noted that killing the being inside, right then, would unwrite a shameful future.Not an evil future. A shameful one. Traitor Primarchs get evil futures. This one got shame.