Fear to Tread
In the novel Fear to Tread, Horus witnessed Sanguinius struggle with and kill one of his Blood Angels, Alotros. When Horus asked Sanguinius the reason he murdered one of his sons, the Angel first refused to answer his brother. Insisting, Horus finally had his answer: Sanguinius transmitted a genetic flaw to his Blood Angels, the Red Thirst, which turns a legionnaire into a mindless killer, leaving him no other choice than killing Alotros. When Horus asked Sanguinius why he never talked about that with his Primarch brothers and, above all, why he never asked for help from their creator, the Emperor, Sanguinius replied he feared the most radical solution his father could propose:
"You know the reason! [...] I will not be responsible for the erasure of the Blood Angels from Imperial history. I will not have a third empty plinth beneath the roof of the Hegemon as my Legions only memorial!"