The central idea of this poem is that regardless of how it happens, humanity is going to destroy itself through its own vices; moreover, this destruction is so inevitable—if we do not change anything—that it is of no use to even get upset about it. The speaker reports that some people say that the world will "end in fire," which he equates with "desire." This could be desire for land, for power, for resources like oil or water, to make another country more like our own: all reasons that we might go to war, employing "fire" with gunpowder and weapons and so forth.
Some people, on the other hand, say that the world will end in "ice," which the speaker equates with "hate." This could be the hatred of people who are unlike ourselves because they are a different race, a different religion, a different ethnicity, and so on: all reasons that we might allow another country or group to suffer through our hate. We might ignore their struggles and allow them to perish, and someday the same could happen to us. "Fire," then, is fast, while "ice" is slow, taking its time, and so the speaker says that he agrees with people who say "fire" will be responsible for our demise. However, he ends the poem with an understatement that underscores its irony: ice, or hate, would more than "suffice" to destroy the world, as it is just as destructive as fire or desire—it just takes longer.