When I was a much younger lad (playing in the Rogue Trader and Realms of Chaos era), I must confess that I had a bit of a different take on the Horus Heresy. The impression that I got from the source materials was that:
(a) the traitor primarchs were the primary people responsible for turning their legions to chaos;
(b) the same primarchs were unrepentant / unapologetic about this.
As I've grown up and read some of the newer materials, my opinion has shifted somewhat on a number of primarchs and their legions. Below, I present brief (one line!) comparisons in the old background (Realms of Chaos) versus the more updated.
III: Emperor's Children: One of the units brought to "pacify" Horus. Horus introduced Slaanesh worship to their highest officers and the legion declared "neutrality". But that wasn't enough for Horus and he waited for the Slaanesh worship rot to spread through the entire legion before they broke and succumed to Horus' side. Most certainly, Fulgrim did not appear to be possessed - he willingly embraced Slaanesh.
IV: Iron Warriors: surprisingly little is said of the Iron Warriors in the old Realms of Chaos books, but there is a small paragraph on Perturabo's daemon world and a couple of imposing banners.
VIII: Night Lords: Dedicated to Khorne according to Slaves to Darkness, but little more is made of them.
XII: World Eaters: Horus (possessed) readily brought them to the worship of Khorne, with little modification of the chapter rituals, bloodsoaked dues to their neural implants as they already were. Perhaps not too much difference here. Angron "genuinely thought that he alone could save humanity from destruction" for his temptation to chaos (Lost and the Damned).
XIV: Death Guard: Mortarion openly offered up the Death Guard to escape the plague whilst they were stuck in the Warp. (Only later did it become apparent that Typhus was the cause of this and secretly aligned to chaos all along). He "fully believed that he was the herald of a new age of justice" in his temptation to chaos (Lost and the Damned).
XV: Thousand Sons: Similar to the modern background. The Thousand Sons were loyal, but practiced sorcery (to the extent of doing so in favour of fighting the reunification wars according to the Lost and the Damned). They tried to warn the Emperor through sorcery of Horus, but got the Space Wolves sent after them for their trouble. Ultimately, they were forced to ally with Horus.
XVI: Luna Wolves: Horus started the rebellion and influenced all the other legions to turn to chaos (rather than the Word Bearers being the cause of it all). Horus instigated "warrior lodges" in to 5 legions apparently (source = Slaves to Darkness).
XVII: Word Bearers: They weren't the first: The Sons of Horus were. Otherwise, very little is said about them in the old realms of chaos books.
XX: Alpha Legion: "Twisted minds and bodies" in the service of chaos according to Slaves to Darkness, and they most definitely were not secretly serving the emperor's aims.