But they didn't kill each other. Even before the Heresy, Leman Russ had battled his own brothers - namely, those belonging to the Legions of those Primarchs who-shall-not-be-named.
It's entirely conjecture on my part, as they likely ended up turning to Chaos before Horus (probably without any sorcery involved, simply because that was part of what made Horus nigh-unstoppable) did. However, given that there are things in 40K that even the C'tan cannot explain, I like to think that the two Unknown Primarchs were either converted to, or simply bore witness to... something.
I was talking about the pre-Heresy time, I never said those 2 Chapters were deleted after the Heresy.