Originally Posted by Malus Darkblade
(especially seeing a Chaos Marine turning into Gollum and having the capacity to feel fear)
That never particularly bothered me, to be honest. CSMs don't have the And They Shall Know No Fear
USR on the tabletop for good reason: they are the Space Marines that felt fear, whose conditioning was imperfect; they were the flawed and the broken, the ones who could be tempted into damnation; they were the selfish and the greedy, who discarded discipline and duty for self-absorption and base survival. If the mind of one of these failures cracks and he's reduced to a gibbering wreck, well...
(there are some spoilery extenuating circumstances that justify some of the things that go on in this regard, too, but those aren't revealed until practically the epilogue, so I won't go into those)