Bane of Empires
Yes, and despite all of that, there was some measure of general acceptance that most of the Legiones Astartes were vital to the war effort and were honourable warriors and unleashed their ferocity only as a last resort. However, regardless of what the individual worlds whom Magnus brought into compliance (and thus had direct dealings with the XV Legion) thought, the vast majority of the Imperium hated and feared psykers, there is no escaping that fact. The Thousand Sons were a Legion of psykers whom even several of the Primarchs condemned, and eventually the Emperor brought to trial and sanction.Sorry, I have to remain in disagreement with you on this.
First, when it comes to the reputation of the Thousand Sons in regards to the rest of the Imperium we have next to no evidence that the planets they brought into Compliance were disposed against them after the fact (anymore so than other Legions that is). I doubt it's an accident that we know for a fact that people were terrified of the Night Lords and the World Eaters, but that no such mention is made for the Thousand Sons.
To this, I'd add the purely theoretical: Astartes warfare has been repeatedly qualified as shocking, extreme, and devastating in nature. We're talking about orbital bombardments that are equivalent to nuclear detonations or to fire bombings of extreme scale, Titans and super-heavy tanks that can level buildings with every strike, and Space Marines who dispatch their opponents in the most gruesome ways: their chainswords eviscerate and amputate, their bolters explode bodies from the inside out, their punches and kicks collapse torso and skull alike, etc. What am I getting at with this? Absent truly obscene displays of psychic power (and we don't get to see this even in the Shrike campaign, in A Thousand Sons, which we know to have been heavily contested), I sincerely doubt that the overt abilities used by the Thousand Sons (telekinesis, pyromancy, etc.) would have made a significant impact on conquered populations - save, of course, for those with the most virulent anti-psyker traditions.
I don't think it matters exactly what the Thousand Sons did - whether they produced "truly obscene displays of psychic power" or not, people perceived them as being witches dabbling in sorcery and thus I still think they would have produced similar amounts of (if not more) "bad PR" than the World Eaters or Night Lords.
Actually, there was. The Night Lords and World Eaters were recalled to Terra to, maybe, be relieved of command or otherwise sanctioned. Yet the Thousand Sons (before Magnus was discovered) were apparently (rumoured) close to being purged. For example, in Ahriman's words:Second, the Council of Nikaea was predicated on the ideological differences between a minority of the Primarchs. There's a huge difference between a parallel to an ecumenical council with hostile overtones and being recalled to Terra to be relieved of command. Before Magnus breached the wards of the Imperial Palace, there was no talk of sanctions as with the other two legions we're talking about.
"They [Mortartion, Corax & Dorn] feared what we could do, and spread their lies to anyone who would listen that we were witches practising unclean sorcery. Little did any of them realise they were condemning the very powers that allowed them to travel between the stars or spread the malicious rumour-mongering."
"...but more and more of us were falling prey to it [the flesh change] and the voices of our persecutors were growing ever more strident. There was even talk of disbanding us and expunging us from Imperial history."
From re-reading the passage in A Thousand Sons, it doesn't seem that the Flesh Change debacle was played out privately or in secret, the wider Imperium was seemingly aware of it and condemned the Legion at every opportunity (apparently not helped by "rumour-mongering" and people of influence spreading such rumours to "anyone who would listen"). It's not necessarily about exactly what the Thousand Sons did or whether they produced obscene/enviable displays of psychic power, they were a Legion of witches who had almost been wiped out due to warp-corruption and who drew huge amounts of condemnation along the way. People hated and feared them because of what they were, regardless of what they did. Thus people's perceptions of them, I would argue, created "bad PR" for the Great Crusade.
For example, a newly compliant world who has just been rescued from the psychic depredations of Old Night joyfully embraces the Imperial Truth and the Emperor's rule. Yet, they learn that one of the eighteen Legions that are spearheading this Crusade to free humanity are themselves a coven of psykers, who this world (and indeed most of the Imperium) perceives to have caused Old Night in the first place. I imagine in this scenario, the Thousand Sons would have been condemned to a greater extent (especially after the Flesh Change debacle which effectively proved that they were unstable and corruptible) than the rumoured atrocities/genocide committed by two of the other Legions. The issue of witches and psykers was one much more prominent given the events of the Age of Strife than of a band of warriors trying to slip the leash and commit a few genocides here and there. Most human worlds in the galaxy had only recently suffered because of rampant psykers (or at least they perceived to have suffered because of psykers - because they blamed the Age of Strife on them) and thus I would suggest this was a much greater issue and cause for "bad PR".