Out of all of the criticism of ADB I have never heard of this one suggesting that he is against marines from traitor legions staying loyal. Now if it was a post-heresy chaos marine that suddenly had regrets, he'd probably have the guy executed by Imperials, because at that point, chaos marines are too far damned to be redeemed.
But to add an opinion on the Blood Ravens, while I believe that Blood Ravens are descended from Thousand Sons, it is also really fucking obvious. What if they were descended from the 2nd or 11th legions? That would be a curveball and possibly explain why the Blood Ravens thirst for knowledge. Maybe one of the Lost Primarchs was a knowledge seeker and found something man was not meant to know?