Board Thread:Warhammer 40k General Discussion/@comment-69.143.134.163-20150419161351/@comment-26516936-20150621032610

The lost primarchs are called the purged and the forgotten which if the 2 words are meant separately for each primarch it would indicate somewhat separate causes for each of their falls. Given how much Angron & Curze were allowed to get away with & what brought about Magnus & Lorgar being called on the carpet we can infer that what ever caused the lost primarchs downfalls was much more likely to do with there blatantly disobeying, violently disagreeing or betraying the Emperor rather than anything to do with how nasty or destructive they were.

Now we know that Russ & the Space Wolves were called on to destroy at least one primarch before Magnus but we have no definitive evidence that Russ destroyed both. In one of the Horus heresy books there is an imperial timeline that has 2 separate incidents a few years apart listed as "data redacted" shortly before Corax was found, the inference being that the two redacted entries were when the lost primarchs fell.

All this seems to indicate that the 2 lost primarchs were brought down by 2 separate reasons & that while 1 of them was crushed by the Space Wolves the 2nd might not have been & might have been for example "forgotten" for, say, quitting the great crusade after violently disagreeing with The Emperor over some part of his policies.