Whenever the forces of Chaos want to turn someone against the Emperor, they show their target a vision of 'the Emperor's true plan'.Partly jossed later in the series Sigismund admits to Garro that he (still) cares about ensuring that Keeler and her message survives.Sigismund was not rewarded, he was exiled, and the eternal crusade is his attempt to atone in the eyes of Rogal Dorn. This explains why Sigismund was made High Marshal of the Black Templars and so much of their psychology: their hatred of psykers, the 'No Remorse' part of their Battle Cry, and the ten thousand year crusade.

In The Crimson Fist short story from Shadows of Treachery, much of the Black Templars mindset is explained when Sigismund reveals to Rogal Dorn that Euphrati Keeler told him he needed to stay with Dorn at Terra instead of lead the Retribution Fleet to Isstvan, and Dorn effectively disowns him as his gene-son in a rage for questioning his purpose and believing he knew better than the Emperor, making him no different than Horus.

Horus heresy tv tropes