Its probably not much but these are my 2 cents.

Word Bearers

With the word bearers its good to note 2 things, 1 they where extremely loyal to Lorgar. And 2 they where the 1st legion by a long margin to actually turn traitor, so they had plenty of time to weed out. As more of a side note, they did purge weaker parts of their legion during the battle of Calth and the shadow crusade but that's quite far past Isstvan 5.

Night Lords

The night lords pretty much went rogue as a legion. And I'd suggest that their actual reasoning of going heretic are fairly different than the other legions. It is noted however that in the short story Massacre by Aaron Dembski-Bowden that the Night Lords did turn on their weak/Loyal brothers whilst enroute to Isstvan 5. And they probably killed more during the dropsite massacre itself but i don't know any proof of that.

The deck gave a violent shudder at the cry of distant thunder. Broadsides, thought Talos. The Covenant of Blood was firing upon its own fleet. That, at last, made him draw breath to question what was taking place. ‘Why?’ he asked, turning to meet his sergeant’s eyes. Anrathi grinned more than most of his brothers. He did so now, bearing his elegantly filed teeth. He didn’t need to ask what the Apothecary was questioning. ‘Because I ordered it, and Captain Malcharion sanctioned it.’ ‘Why?’ Talos repeated. Irritated curiosity narrowed his eyes. He wanted answers, not another of Anrathi’s dances around semantics. ‘If we kill them now,’ the sergeant replied, ‘we don’t need to kill them later.’

Iron Warriors

The Iron warriors where quite a split legion. However when word got out that Olympia turned heretic, Perturabo went for a mass assault on the planet millions of civilians where killed. but more notably A part of Iron Warriors resisted the orders from Perturabo seeing how crazy and cruel they where, and they where gunned down by their own battle brothers. Though the legion as a whole was really stunned at this. It quite shows what they would do for their primarch and it probably spiraled do quite fast. which is described in Perturabo's Primarch novel.

Alpha Legion

For the alpha legion I don't quite know, it's complicated. It is said that they where still loyal to the emperor, though with Alpharius disagreeing with Omegon on how to help the imperium by siding with Horus, Alpharius more willing to sacrifice and Omegon trying to do as little damage to the imperium as possible. So I can't fully say what's up with them other than it's complicated....

It's not much, but here are my 2 cents.

Word Bearers

With the word bearers its good to note 2 things. 1 they were extremely loyal to Lorgar. And 2 they were the 1st legion by a long margin to actually turn traitor, so they had plenty of time to weed out. As more of a side note, they did purge weaker parts of their legion during the battle of Calth and the shadow crusade but that's quite far past Isstvan 5.

Night Lords

The night lords pretty much went rogue as a legion. And I'd suggest that their actual reasoning for going heretic are fairly different than the other legions. It is noted however that in the short story Massacre by Aaron Dembski-Bowden that the Night Lords did turn on their weak/Loyal brothers whilst enroute to Isstvan 5. And they probably killed more during the dropsite massacre itself but i don't know any proof of that.

The deck gave a violent shudder at the cry of distant thunder. Broadsides, thought Talos. The Covenant of Blood was firing upon its own fleet. That, at last, made him draw breath to question what was taking place. ‘Why?’ he asked, turning to meet his sergeant’s eyes. Anrathi grinned more than most of his brothers. He did so now, bearing his elegantly filed teeth. He didn’t need to ask what the Apothecary was questioning. ‘Because I ordered it, and Captain Malcharion sanctioned it.’ ‘Why?’ Talos repeated. Irritated curiosity narrowed his eyes. He wanted answers, not another of Anrathi’s dances around semantics. ‘If we kill them now,’ the sergeant replied, ‘we don’t need to kill them later.’

Iron Warriors

The Iron warriors where quite a split legion. However, when word got out that Olympia turned heretic, Perturabo went for a mass assault on the planet millions of civilians were killed. but more notably A part of Iron Warriors resisted the orders from Perturabo seeing how crazy and cruel they where, and they were gunned down by their own battle brothers. Though the legion as a whole was really stunned at this. It quite shows what they would do for their primarch and it probably spiraled do quite fast. which is described in Perturabo's Primarch novel.

Alpha Legion

For the alpha legion I don't quite know, it's complicated. It is said that they were still loyal to the emperor, though with Alpharius disagreeing with Omegon on how to help the Imperium by siding with Horus, Alpharius more willing to sacrifice and Omegon is trying to do as little damage to the Imperium as possible. So I can't fully say what's up with them other than it's complicated....

Its probably not much but these are my 2 cents.

Word Bearers

With the word bearers its good to note 2 things, 1 they where extremely loyal to Lorgar. And 2 they where the 1st legion by a long margin to actually turn traitor, so they had plenty of time to weed out. As more of a side note, they did purge weaker parts of their legion during the battle of Calth and the shadow crusade but that's quite far past Isstvan 5.

Night Lords

The night lords pretty much went rogue as a legion. And I'd suggest that their actual reasoning of going heretic are fairly different than the other legions. It is noted however that in

Iron Warriors

The Iron warriors where quite a split legion. However when word got out that Olympia turned heretic, Perturabo went for a mass assault on the planet millions of civilians where killed. but more notably A part of Iron Warriors resisted the orders from Perturabo seeing how crazy and cruel they where, and they where gunned down by their own battle brothers. Though the legion as a whole was really stunned at this. It quite shows what they would do for their primarch and it probably spiraled do quite fast. which is described in Perturabo's Primarch novel.

Alpha Legion

For the alpha legion I don't quite know, it's complicated. It is said that they where still loyal to the emperor, though with Alpharius disagreeing with Omegon on how to help the imperium by siding with Horus, Alpharius more willing to sacrifice and Omegon trying to do as little damage to the imperium as possible. So I can't fully say what's up with them other than it's complicated....

Its probably not much but these are my 2 cents.

Word Bearers

With the word bearers its good to note 2 things, 1 they where extremely loyal to Lorgar. And 2 they where the 1st legion by a long margin to actually turn traitor, so they had plenty of time to weed out. As more of a side note, they did purge weaker parts of their legion during the battle of Calth and the shadow crusade but that's quite far past Isstvan 5.

Night Lords

The night lords pretty much went rogue as a legion. And I'd suggest that their actual reasoning of going heretic are fairly different than the other legions. It is noted however that in the short story Massacre by Aaron Dembski-Bowden that the Night Lords did turn on their weak/Loyal brothers whilst enroute to Isstvan 5. And they probably killed more during the dropsite massacre itself but i don't know any proof of that.

The deck gave a violent shudder at the cry of distant thunder. Broadsides, thought Talos. The Covenant of Blood was firing upon its own fleet. That, at last, made him draw breath to question what was taking place. ‘Why?’ he asked, turning to meet his sergeant’s eyes. Anrathi grinned more than most of his brothers. He did so now, bearing his elegantly filed teeth. He didn’t need to ask what the Apothecary was questioning. ‘Because I ordered it, and Captain Malcharion sanctioned it.’ ‘Why?’ Talos repeated. Irritated curiosity narrowed his eyes. He wanted answers, not another of Anrathi’s dances around semantics. ‘If we kill them now,’ the sergeant replied, ‘we don’t need to kill them later.’

Iron Warriors

The Iron warriors where quite a split legion. However when word got out that Olympia turned heretic, Perturabo went for a mass assault on the planet millions of civilians where killed. but more notably A part of Iron Warriors resisted the orders from Perturabo seeing how crazy and cruel they where, and they where gunned down by their own battle brothers. Though the legion as a whole was really stunned at this. It quite shows what they would do for their primarch and it probably spiraled do quite fast. which is described in Perturabo's Primarch novel.

Alpha Legion

For the alpha legion I don't quite know, it's complicated. It is said that they where still loyal to the emperor, though with Alpharius disagreeing with Omegon on how to help the imperium by siding with Horus, Alpharius more willing to sacrifice and Omegon trying to do as little damage to the imperium as possible. So I can't fully say what's up with them other than it's complicated....