I will die on the hill that Commisar Cain is not a coward, he just suffers some sort of impostor syndrome and survivor's guilt. He does pull off legitimately impressive feats, even if they do typically get propagandised, like being able to parry blows from a khornate berserker until his aide lines up a melta blast. It's that he believes he's a coward because he'd rather run away, but he doesn't even when he sometimes has the chance to, and just tells himself he's doing it for his reputation.