Death

Death really takes over the story after the baby is delivered. Nick's dad's whole intention for the trip—getting his kid to see a live birth, which Nick is reluctant to do—becomes reversed when Nick ends up seeing what his father tries to hide from him. Take a gander at his reactions to first the birth, and then the suicide:

He was looking away so as not to see what his father was doing. (28)

Nick, standing in the door of the kitchen, had a good view of the upper bunk when his father, the lamp in one hand, tipped the Indian's head back. (45)

It's unclear here whether Nick looks at the suicide out of his own desire to see, or whether he accidently happens to get a full-frontal view of it. Given how squeamish Nick is, we can probably assume the latter, though it may be that Nick is more fascinated by the idea of death. We definitely get a sense of this from the awkward father-son talk on the boat ride back, in which Nick's questions are all about death and suicide instead of about the birth he just witnessed.

So death seems to win out in the end, but the question now is why. There isn't a hard, fast answer to this one. Instead, there are a few routes we could take: we could think about not just death, but the idea of suicide and how that corrupts a clean-cut view of the world; we could think about how all people end up contemplating their own mortality, and now Nick has been initiated into that adult world; we could think about how Nick's father is so eager to shield Nick from the suicide, even though he was pretty adamant about him watching the birth. See, death was just the beginning.