Some say the world will end in fire, Some say in ice. (lines 1-2)
The poem abstracts itself from any specific vision of the end of the world.
But if it had to perish twice, (line 5)
Really? The world has to end twice so we can test out the fire vs. ice hypothesis? In all seriousness, the speaker is dripping with irony here. He takes a terrifying topic and treats it like a rational thought experiment.
I think I know enough of hate (line 6)
Maybe the speaker's lack of fear has to do with his experience in matters of desire and hate. He has seen enough destruction in the world – or at least destructive emotions – to not be afraid of it.