Fire and ice
Fire: It is the symbol passions for love, hatred, power, money ,etc. or it can be the symbol desire(instinct) according to the poem.
Ice: Is the symbol of being cold and indifferent towards everything except ourselves.
The scientific view about the end of the world.
1.In the future the earth is gonna be away the sun so the existence is gonna be finish in ice because when the earth is away from the sun , it will be frozen.
2. some other scientists believe that the earth is gonna be exploded because having too much heat and energy in its core.
The attitude of the speaker of the poem is he believes in the ending of the world in fire.
(lines3-4) he has experienced fire which is the symbol of passions for love, power, hatred, ambitions, beauty ,etc. It means that he thinks human beings are mostly busy with fighting for their passions.
(lines5-9) according to the speaker if it's possible to live twice, there will be enough fire of hatred , wars, power, etc to melt the ice of indifference and coldness among the people. So again he emphasized that he believes that the world will be finished in fire.
Perish: This world has a biblical allusion. That is , it refers to the story of mankind on the earth which will be finish with man's body to be perished. We have perishment for the body while the soul will be survived.
Twice: This word can be metaphor for reincarnation. The Buddhist and Hindu people who have the Oriental Religion believe that human being can live several lives. But such a theory has been rejected by the divine religions.
Fire and ice =oxymorn