His experience with Space Origin had left a loophole in his understanding of Origin. Space Origin had not been fully claimed by a Primordial.
So he was not aware of the numerous changes that could have been made when they had fully inherited an Origin.
Rowan obviously did not know that there could ever be remnants of past soul origins inside the Origin he was devouring, and he was angry at himself for this apparent oversight.
He had seen how Origin was able to preserve the remnants of life after the entirety of a being had been destroyed; why should he not consider that it should be able to hold other dead entities that had no reason to be alive?
Rowan's struggle came from the fact that he was presently consuming the Soul Spiders, which drained a lot of his consciousness powers, making it the perfect time for these four Echoes to possess his soul.
