Board Thread:Warhammer 40k General Discussion/@comment-6078851-20130130174042/@comment-101.113.141.184-20130201232042

Sanguinius, Ferrus Manus, Horus Lupercal and Konrad Curze are the dead known primarchs - conveniently 2 of each from order and chaos to keep everything even.

Also with the missing primarchs/legions it was designed by games workshop to allow custom space marine chapters.

In the Horus Heresy it is known by references that Russ has been called on by the emporer to deal with at least one of the primarchs. Being dubbed The Emporers Executioner by a couple of his brothers can also lend claim to this, though unconclusive)

In the book (I forget the name) in which the Blood Angels are sent to Signus Prime. There are sections depicting the past in which Horus saw Sanguinius kill one of his sons who had fallen to the Red Thirst. When Horus asked why he didn't inform the Emporer, Sanguinius had told Horus that he wont allow what happened to his lost brothers legions to happen to his due too a mutation/flaw in the gene seed. (Sanguinius believed him and his legion would be wiped out from history if the Emporer knew of the flaw.)

^ From that passage it would indicate that either the primarchs or the sons had dangerous and unstable mutations. This does NOT mean it was from the influence of Chaos, just a design flaw.