It wasn't much fun, but the only one to whimper was Sam McGee. (line 16)
This is just a quick remark by the speaker, but we think it might say something about Sam. You need to be tough as nails to survive in the Arctic winter, and you probably shouldn’t be doing a lot of whimpering. This line makes Sam sound a little weak, frankly. Now, the poor guy’s freezing to death, so it’s hard to blame him, but we think it’s safe to say that he’s lost the grit and determination he needs to get by.
With a corpse half hid that I couldn't get rid, because of a promise given; (line 30)
Our speaker makes it clear from the start that he isn’t going to give up on his mission, even if it’s tough on him. And it is tough. He tells us plenty about the disgusting smiling corpse, and how much he hates lugging it around. We admire him even more for hanging tough even when the job is so gross and thankless.
Now a promise made is a debt unpaid, and the trail has its own stern code. (line 33)
This little bit about the code of the trail seems important to us. He doesn’t say a lot about it, but we think it shows why he’s so determined. Our speaker is driven by a sense of honor and loyalty. In a place where there’s no one to keep tabs on you, there’s still an unspoken rule about how you’re supposed to act. Maybe this code helps to keep things together, and helps to keep the men sane, too.