Could it be that the reason Nathaniel Garro and the other loyalists that survived Istvaan III, were permitted back in the fold, was because the Imperium of old operated differently? I'm just venting a thought here but:
In the Imperium, just before/during the Horus Heresy, the Horus Heresy was in fact not a matter of heresy, was it? It was an act of treason on the highest scale, but seeing as how the Emperor was outlawing religion, the idea of it being heresy was likely brought in with the rise of the Imperial Cult, later.
So the traitor legions were considered just that, traitors. When some of them returned, apparently not having turned traitor, but in fact having slaughtered their way through so many traitors, to get back and join their loyalist brethren, accepting them back in the folk was reasonable.
Now, 10.000 years later, every traitor chapter is considered guilty of heresy, and the Imperium have a tendency to find people guilty of heresy-by-association.
If you are excommunicated from the Imperial cult, for any reason, it's open season on your ass.
Loyalists marines who have fucked up and want to redeem themselves can become Blackshields, but I have not heard of traitor.-legion marines, being permitted this, or even being permitted to raise their voices in defense, without getting a bolt-shell in the teeth.
I am sure attitudes have changed in the Imperium since the time of the Emperor but in those days, genoicide and casual murder was as much a tool as it is in 41,000. Garro was not allowed in the presence of the Emperor, he was sent to Luna to be investigated for taint. So there was already an appreciation of the true nature of the traitor legions.
As to the black shields, I have all the DW books and I don't seem to remember (doesn't mean it isn't true, my memory isn't what it used to be) that the DW is used as a dumping ground for fuckups. It is an elite organization and to be detached to the DW is a matter of great pride and honour for a Space Marine and his parent chapter. And I also can't recall SM's undergoing punishment having to relinquish their armor and identity. The whole notion of black shields is that they are unknown space marines, Except for the watch commander who accepted them, they could be anyone. From a destroyed chapter, from a rogue chapter or maybe even from a traitor legion. In the latter case, probably not an original marine since they are likely too mutated and too tainted to be accepted even if they wanted to switch sides but the legions recruit new marines who may not want to stay….
in fact they are. by some ironics jokes, logar wrote the "lecto divsionis" the book about his faith in the emperor as god wich has been the base of the imperial creed before being judged by the emperor and turning to chaos.
Except that no member of the Imperium, let alone a member of the Inquisition, in the 41st millenium will have any idea of the origins and founding members of the Holy Ordos. There have been more years past from then till the 41st millenium than we have recorded human civilisation as of 2012, and look at how little we have kept true to from our ancestors!
I think that without some incredibly (read half a dozen degrees of success) good Forbidden Lore:Inquisitor and Forbidden Lore:Traitor Marines/Horus Heresy (depending on what is in your game), that it'd be impossible to use the meta-game knowledge in character. Even then, trying to justify your conversing with the Word Bearers to your superiors would be little short of out-right treachery and heresy.
Now, if you have a psyker in the party who can read minds, or if you have some sort of restraints suitable for an astartes such as a stasis casket or field shielded cage then it may be possible to 'interrogate' one of these ancient warriors without specifically attacking them and thus betraying their supposedly honest intentions…
Wait… what? First you say "nobody knows" then you say "but if you get a great roll you do"
You're supposition is just as assumed as mine in that department, neither of us know what of the inquisition (or the greater imperium) knows about its past. I was just showing that obviously being from a traitor chapter isn't a death sentence in the imperium and in fact those who resist the call of chaos turn out to be some of the imperium's greatest champions.
No, what i said was that without rolling well ingame, you cannot just use your personal knowledge of the setting and history of the setting and presume your character would know it. However, even with good rolls ingame it would not be permissable to just say "oh yeah, these guys are fine, lets hang out with them!"
"Only the insane have the strength enough to prosper. Only those who prosper may truly judge what is sane."
Point of fact: The word bearers are arguably the true "founders" of the modern Chaos legions. It was Erebus, A Word bearer chaplain, that arranged for the corruption of Horus. Even If I "Knew" the history of this particular chapter they would be extremely suspect! That being said; In the days of the great crusade a Space marine legion was composed of thousands of Astartes and likely not all of them fell to the ruin of chaos! Original era Word bearers would likely feel vindicated that the Emperor was finally recognized as a god! It is up to your GM to determine where this particular band fits.
The Emperor protects! (The GM does not!)
How do you know they are telling the truth?
Even before the Heresy the Word Bearers were engaged in the build up. Big question would be what colour their armour is. If its grey they might be telling the truth but if its red then they are almost certainly traitors even if they have been in statis/whatever for the last 10,000 years.
All of which is useless because your characters wouldn't know that but still the point is valid, how do you know whats actually happening. It then depends on the characters to decide what to do based on their leanings. One option would be to try and get them to join the Deathwatch as Black Shields, another would be to keep them hidden away as your very own Astartes unit. A good puritan who found out they were members of a Traitor Legion would likely purge them in case it was a trick or in case they decided to follow their brothers into heresy.
In the end its up to you. I would never be able to trust them personally, the what if scenarios would be horrifying to imagine. It would take someone of massive arrogance or hubris to think they could control Astartes nevermind potential chaos marines.
In general I would imagine the Imperium would not be comfortable with marines that would outright contradict its version of events so I imagine it would be only a matter of time before one faction or another was after them even if they were completely loyal.
As usual, the best answer would be: “it depends”.
Are Space Marines rock stars in your game whose exploits are lauded on human worlds with space marine dolls in correct chapter colours sold to children or are they mythical super warriors whose presence is only mentioned in prayers and fables? That would namely determine how much would be known about specific Astartes history.
This is probably the best answer.
looking back the Word Bearer's Armor colors and icon has also changed from pre-heresy to post heresy considerably. I'd think it would incredibly hard to "know" who a traitor legion is without a the setup like Ranoncles said. I mean "we" know who they are but the characters in our universe? I think it would entirely depend.
If you found a random set of non-chaos designed space marines fighting clearly chaos aligned ones, I think your first reaction (besides holy emperor! space marines!) would be that this is some random extra-founding (I mean there's what? a thousand chapters?) Our suspicions only arise because ooc we know the background of the word bearers
"Silence you furry fool! I've had it with your whiningbumbling. You're finished here, cast out! Banished!"
~Lord Inquisitor Skeletor to Acolyte Beastman
Sisters of Silence Suppliment:
It's SO funny that the Word Bearers and particularly Lorgar and Erebus are responsible for both the Horus Heresy as well as the whole current state of the Imperium (Adeptus Ministorum and all that), since Lorgar wrote the "Lectitio Divinitatus" that became the seed of the Imperial Creed, and later on they turned Horus to Chaos =^_^=
Suffer not the Alien to live !… Of course do thoroughly interrogate it first.
Inquisitor Qintus Minos.
I think this discussion shows why we like this Warhammer 40-Setting and especially the DH-Setting so much: no clear lines between good and evil; the universe is not simply black or white - there are only shades of grey; good people with good intentions do good things which lead to disaster and chaos …
Concerning the Wordbearer: In the end it all depends on your inquisitor. Is he a zealous and puritan guy? Then he will probably try to eradicate the SMs. Is he more on the radical side? Then he might see them as a tool for his own goals. (Personally I would assume that even the most learned inquisitor wouldn't know the names of many traitor legions or chapters. The chance that your inquisitor would ever have heard of the Wordbearers would be infinitesimal. For the common man traitor legions are more kind of a legend like, say, "Lucifer the fallen Angel" for us.)
A good example for the frictions between the Adeptus Astartes and the Holy Ordos is "Emperor's Gift" - Dembski-Bowden's last Grey Knights novel: an inquisitor trying to subdue a Space Marine chapter (Space Wolfs), SMs battle other SMs, inquisitors battle inquisitors - all of them with "good" or at least legitimate intentions. All of them weakening the strength of the Empire with their actions…
And all we hear in the end is "… the laughter of thirsting gods".