Line 25

A pal's last need is a thing to heed, so I swore I would not fail;

• The speaker considers Sam his "pal," so he feels like he pretty much has to do the right thing and help him out. He swears he’ll burn Sam's body when he dies.

Line 26

And we started on at the streak of dawn; but God! he looked ghastly pale.

• They continue with their trip at dawn, but Sam isn’t looking so good; in fact he’s "ghastly pale."
• We can tell it isn’t going to end well for Sam.

Lines 27-28

He crouched on the sleigh, and he raved all day of his home in Tennessee;
And before nightfall a corpse was all that was left of Sam McGee.

• Poor Sam sits in the sleigh all day and "raves" (talks crazily) about Tennessee. It’s kind of a sad image isn’t it? This poor, dying man, freezing to death and thinking about his warm home.
• In any case, it doesn’t last long, and Sam is dead by nightfall.