“Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate To know that for destruction ice Is also great And would suffice."
Fire is emotion, ice is hate. Frost leaves it clear that it is either fire or ice that destroys the world, and nothing else. It seems clear that either can destroy.
The two elements have a metaphorical relationship with the "world". Too much fire and passion can destroy a relationship while cold, indifference and hate can do the same. The world symbolizes our relationships and fire and ice represent the various people in our lives.
Some people are fire and some are ice inferring that becoming surrounded by one type can be destructive.
Perhaps "Fire and Ice" was inspired by Dante's Inferno where sinners are condemned to a fiery hell and then submerged up to their necks in ice.