When it came to which Primarch was most likely to turn traitor, everyone had their own opinion. Maybe it was Angron, reduced to a mount by the Butcher's Nails, or Curze, who grew up in a sewer. You could always find one that fit the bill.
But most Primarchs caused all sorts of fucked-up trouble later on because of the environments they grew up in. If you dumped Angron on Macragge, who knew? Maybe he would have become a true Regent. But two of them—and arguably three—were traitors purely because of their personalities.
One was Plankton, all twisted up inside. The other was the perfection-seeking Phoenician, Fulgrim. In fact, compared to Fulgrim, Plankton might have improved after a good beating. Simply put, he was starved for love and needed a good thumping.
As for the last one, that had to be our Rainbow Pony, blessed with peerless wisdom. The Warp Spiritling had him utterly bewitched, and in the end, without thinking it through, he made that fateful psychic call that changed the course of history.
But Fulgrim was different from all of them. His drive for perfection was etched into his very bones. He didn't understand that imperfection was what made something truly perfect. With such a glaring flaw in his character, corrupting him had been child's play.
As true close friends, the Phoenician and Ferrus had gotten along exceptionally well. They were practically a repeat of Guts and Griffith; even their stories had a similar feel. The sacrificed members of the Band of the Hawk silently gave it a thumbs-up, then turned their heads and saw the Iron Hands who had died in the orbital bombardment on Isstvan.
Had he regretted ascending to daemonhood? Of course he had. Fulgrim had indeed fought against the Greater Daemon inside him back