That all makes sense...until Fafnnr says, "Not so undeserved a reputation after all, eh?" What sort of reputation are they deserving of now, suddenly, that they would not have deserved after annihilating two other Legions 50 years before?
Also it is incredibly unlikely the two lost Legions met their ends in two separate tragedies.
How about looking at it this way potentially. The Wolves are sent to sanction the XI Legion, then later on down the line are then ordered to sanction the II. They now get the reputation and title, official or not as the Emperors executioners. To some, such as Guilliman, two separate events form a line, not a pattern, chalk it up to coincidence, Russ and his Legion just might have been in the right place at the right time for both separate events to be chosen. Ergo, Guilliman and others think the reputation is undeserved. But then factor in the Burning of Prospero and you now have three separate incidents, now you have a pattern.
Speculation of course, but makes enough sense to me. Regardless, from all the hints and comments that have been dropped over the series, I'm more than convinced that the Vlka Fenryka carried out the execution of at least one of the missing legions, if not both.
Look to The Outcast Dead
, why would the Gregora(think it was him..) upon learning of that it was Magnus that caused all the chaos say to Kai when asked what would happen next, that "The Wolves will be unleashed again" or similar words to that effect, 'again
' being the key word. Why would the Wolves be chosen by Malcador to send a pack to the side of all the remaining loyalist Primarchs? When Guilliman confronts Lorgar over his betrayal, he rages at him "Pathetic. Our father should have left you out in the snow at birth. He should have fed you to Russ. You worm. You maggot
.", again...why Russ? Why would they be refereed to as the Executioners, or refer to themselves as such long before Nikaea if they hadn't already done an action that would convey that epithet?
Again, from all the material we have from the Heresy series and Forgeworld, I don't need any more convincing.