Some random ramblings about Outer Dark:
It feels to me like Culla's real sins in the book are more of omission than commission. I'll leave out the incest for now, since it happens before the start of the novel. Probably the worst thing he does in the course of the novel is to leave the baby alone in the woods, probably to die. However, we can also look at this as a case of malign neglect -- rather than his actually killing the chap, he leaves it there for, as it happens, the tinker to find.
After that, we have the case where he doesn't put out his hand to help the swineherd as he's swept away in the stampede. And, of course, the scene at the end of the novel (which is what actually led me down this chain of thought in the first place), where he doesn't bother telling a blind man that there's a swamp ahead of him.
I'm not sure how to put this into a context of crime/punishment, which really seems critical to the novel's underlying moral sense. It feels like there's some sort of malign fate at work -- you may not get rewarded for your good deeds, but you'll certainly be punished for the bad ones. Even Rinthy, who's spared much of Culla's worst fate is still unable to be redeemed, as the chap dies before she can ever find him. I think that the milk she expresses through most of the novel is a sort of stigmata -- a representation of the crime that she can't find absolution for -- but it also functions as a goad to drive her to find her son.