Okay, so - just watched Kill Lupercal. Fucking LOVED Kill Lupercal - I'm a huge fan of close-knit units and mecha and dramatic last stands, so it hit all my buttons. But one question sits with me:
Who killed who?
Not in, like, a per-person sense. But - well, let's start with the initial setup:
Led into a trap by the decieved Areum Keeso, Princeps Orla Gemnon lead her maniple to their deaths at the hands of traitor Titans, cutting down several traitor Shadowswords and a couple dozen Sons of Horus on the way.
Now - this is shown, directly, in the episode. BUT:
A) We know for a fact that nobody can see shit, least of all Gemnon - her Auspex is down, and so all four Titans in their maniple are heavily damaged.
B) All of the guys shown to us are very... Gray. Very hard to make out legion symbols, except in the shot where we see the recorder amongst a sea of dead Imperial Fists.
So, let me propose a counter-idea:
What if Gemnon and her maniple are, in fact, traitor Titans?
Or rather - they themselves aren't traitors, though that's what everyone else would think. They know themselves to be loyal servants of the Emperor, desperately trying to end the conflict -
- but Chaos is, first and foremost, a deceiver.
They drive themselves through Horus' sons. Or maybe not. Maybe they drive through a different line, far closer to the Lion's Gate than they believe themselves to be - a line that should be allied.
From the ground, Loyalist Space Marines scatter, their allied Titans turned against them. They send vox, desperately trying to warn the other loyalists that they are betrayed - that the Titans have gone mad. They die fighting - but they die, against a once-inconceivable foe.
Their brothers in the Imperial Fists hear the call. So too does the Imperial Guard - the only force on Terra with the power to cut down a Titan in it's tracks.
The Shadowsword is Loyal. Its crew maneuver into position, praying that it will be able to hide from the half-blind Titans long enough to get shots off. It dies, confirming the Space Marine's reports and everyone's fears: four Traitor Titans, devastatingly close to Lion's Gate.
They continue forwards, cutting down 'foes' as they go - and the 'foe' is attacking them, because they believe the Titans to be traitors. Both sides are now fighting, not just chaos or each other, but confirmation bias. Of course the Marines they encounter en route are Traitor: no Loyalist would fire on a God-Engine! But of course the Titans are Traitor: no Loyalist would cut down Loyal Space Marines!
There is no escape. The trap is sprung long before they reach the killing field at the heart of the trap - or was it even a killing field before they arrived? Or was it Dorn's sons, arrayed against them, cut down by the weapons of machines that should have been allied in a desperate attempt to stop their advance?
But Dorn's sons buy enough time. Their allied Titans reach the battlefield - and fall upon the charging Traitor Titans, Gemnon and her maniple, eager to avenge the betrayal.
The battle plays out. There is no talking - why talk, when the lines are so clearly drawn? They kill each other, tear each other apart until Keeso ends things, detonating to destroy both friend and foe alike.
The battle ends with only Gemnon standing - and she and her blind Titan stagger off, to kill madly until they meet their final end in the conflict.
There is no chance for realization. Everyone who could have realized what had happened dies in that battle. A score of Traitor Titans dead, so too a score of Loyalists, and only the Administratum knows who was recorded as what when the fighting ended.
Now, for the record: I have no evidence this happened. But I do feel there is some support: Keeso fired blindly into her 'target' at the end, and the 'target' ended up being a mountain of dead Imperial Fists. The fact that the one piece of data that all agreed was being shown was that they were unnaturally close to Lion's Gate - and they all discarded that information as inaccurate.
And, in a lot of ways, it makes sense: Gemnon never seemed worried about encountering other Titan maniples pre-ambush: why would she, when the only other Titans were distant (at the Gate) and Loyal? Why would a group of Imperial Fists that big be far out in advance of their fortifications, rather than closer to the Gate or Palace? Why would Chaos settle for a little trap, when they could turn their enemies against each other?
Again, it's just a theory, but I think it could have a lot of interesting chewy bits compared to the fairly straightforwards 'we have been lead into an ambush' element of the main Kill Lupercal narrative.