The tone of the poem is depressing and questionable. As Frost seems like he feels sad that the world is bound to end. But he is also questionable of how it is going to end. Whether it is going to end through hatred towards one another or by other desires and selfishness.
Fire is a symbol for desire. As people can desire for the wrong thing and it can lead to worse things. The ice is a symbol for hate. As a cold heart means cruel.