I haven't read enough yet to decide about the loyalist side. But for the traitors, it's Lorgar and the Word Bearers.
Lorgar because of the choices he made and why he made them. Overall he strikes me as a butthurt 12 year old who's off in la-la land. He was unhealthily obstinate, senseless and uncaring about the consequences of his decisions and utterly naive (not in the cute way). He didn't care about humanity's well-being, he cared about worship - this is clear because of the course he took after Monarchia (ie. worshipping and spreading the taint of Chaos). It seems like he did what he did partly out of spite for the Emperor and being butthurt, the other half because of his screwed mentality that he always had. He didn't respect the Emperor's wishes, just like Magnus didn't, and he had no good reason not to. If it weren't for Lorgar thinking he knows best then he would never have been shamed at Monarchia in the first place. Overall this character strikes me as someone who was fundamentally weak and dangerously deluded being in a position of too much power and free action. He found out the horror of Chaos so instead of cooperating with the Emperor and furthering the fight against it he somehow actually believes that enslaving mankind to the abominable horror of the Chaos Gods and the Warp would be good.
Word Bearers because they went along with it - I don't care how much you like Father Lorgar, ''this is really horrible, oh well, it's the truth so we're doing it'' is still idiotic. I did notice an interesting contrast - there were those who were simply fools like Argel Tal who went along because you know, they were Word Bearers, their Primarch was doing it and it was still the truth even if it was horrible (I don't agree with that sentiment at all but I can see their lawful stupid ''truth is what matters'' point of view) - and then there were those like Xaphen who strike me as being evil, fully understanding the horror they would bring about and actually believing it was not only the truth but good and that they were righteous etc. a really disgusting attitude. Xaphen after he converted to Chaos struck me as quite disturbing, the kind of person you really want to avoid. Not even Erebus despite an architect of Lorgar's turn to Chaos seemed as vile as Xaphen and Lorgar since Erebus doesn't seem to be as pretentious as those two. Erebus is confident, knows what he's doing and why, making no pretense about it, while Lorgar and Xaphen are like really spoiled kids failing at the life lesson that they're not always right.
The First Heretic is the only Horus Heresy novel I've read so far (will start reading them again ASAP and return with more comments it was a really cool and well-designed read) but I don't think I'll find anything more pathetic than Lorgar and the Word Bearers.