Nature, as the antagonist in "To Build a Fire," is not a reasoning or deliberate force, but rather an environment, in which man can live or die regardless of intent. Nature does not care about anything, since it has no mind; the man in nature seeks to tame it with his own power.
...all this -- the mysterious, far-reaching hairline trail, the absence of sun from the sky, the tremendous cold, and the strangeness and weirdness of it all ... did not lead him to meditate... upon man's frailty in general, able only to live within certain narrow limits of heat and cold.
(London, "To Build a Fire," eNotes eText)
The man's refusal to treat nature as a force that can and will kill him without concern is his downfall. Nature exists beyond the scope of reason; it has no structured purpose, only existence. The extreme cold in the Yukon is suited to some animals, and unsuited to others; the dog can survive without tools, but the man cannot. The cold that kills the man is not a weapon, but a fact of life; nature, in this sense, is a place where man invades, a place where man is not meant to be.