Lorgar
Back in the day Lorgar loved the Emprah. Like really loved him, in fact he was the first primarch to venerate the Emprah like a God. He loved him so much that he wrote an awesome book about how great he was and built a fuckhueg cathedral to him, along with building fuckheug cathedrals in every last world the conquered for humanity. Empy caught wind of all this and was not amused, since Lorgar's religiosity threatened to undermine his super sekrit operation starve-the-fuck-out-of-chaos. He made Lorgar tear it all down, upon which Lorgar decided love was over and found himself some gods that could really appreciate his stalker like adoration. So now he's the daemon primarch of the Word Bearers, the guys who make the rest of the chaos space marines look like all around swell secular humanist types. They're kind of like Jehovah's witnesses except instead of knocking on your door and telling to you about Jesus they knock down your door and ask you which end of a demon summoning you want to be on. This tends to make chaos's otherwise awful selling pitch seem oddly compelling.
Since the heresy Lorgar's notable achievements consist of becoming a demon prince of chaos undivided (lolwut?) and then doing sweet fuck all for several millenia. Seriously, at least Magnus pretends to have a plan while he sits in his tower screaming, "JUST AS PLANNED". Lorgar? The lazy fuckwit has just been holed up on Sicarus meditating and traversing the warp. Congratulations chaos gods, you created the lovecraftian equivalent of that friend who won't get off your couch.