I forgot to mention this during Ruinstorm, but it's very very important in my opinion.
A big theme in Ruinstorm is temptation. All of the primarchs must go through temptation in order to survive the ordeals the story puts them through and escape the Ruinstorm. The big bad is Madail, a daemonic evangelical preacher (now I know who's possessing Kenneth Copeland, thanks David!), he's preaching the Bad News of Chaos to the masses, and the brothers must resist the lure of his unholy attractions. And of course, all of the lures are tailor-made to suit the brothers.
Guilliman--ever the practical man--is lured by the promise of greater capability. Early in the book, he comes into possession of a pair of athames. He realizes that these blades have some significance to Chaos, and thus decides to hold onto them. Could he learn more about Chaos's motives and methods from these blades? Could he even utilize them against Chaos? Eventually he realizes that the blades themselves are the lure--holding onto them and studying them will eventually lead to his fall. Can't have that happening! So Guilliman crushes the blades with the Hand of Dominion.
Sanguinius, haunted by visions of his own death, is tempted by his survival. He lives through his slaughter at Horus's hands over and over and over again--but hang on, he wins one of those fights! And then he wins again! Maybe there's a future where he defeats Horus and doesn't die! At the book's climax, Madail tells him that the only way for him to win and survive is to embrace Chaos. Sanguinius turns him down, grieving his imminent death and the Black Rage it will inflict on his sons.
And Lion? What is Lion tempted by, you ask?
Lion is tempted to fix all his problems by Exterminatus-ing Davin, even though Sanguinius has gone to the surface with Curze.
WARCRIMES EL'JONSON MOTHERFUCKERS!!!!!!!
