This idea of the world ending ("perish[ing]") twice is complicated. If we don't even know if the world will end once, why worry about the second time? Would anyone even be around to see to second ending? If fire is more powerful, why wouldn't the world just end in fire twice?
OK, take off your Skepticism Hat and put on your Imagination Hat.
The idea seems to be that "fire" beats "ice" to the punch in the first round of the battle.
Remember that those "hot" emotions are the impulsive ones. If "fire" refers to the animal side of our nature, then the actions associated with it are instinctual – we don't think, we just do.
Icy actions require thinking and deliberation. So, we'll see what happens in Round 2.
I think I know enough of hate To say that for destruction ice Is also great
The speaker has also experienced hate, a cold emotion, and he knows it can destroy things – namely, the world.
At this point, it becomes even clearer that "desire" is associated with "love," because its opposite is "hate."
You'd be hard-pressed to find two emotions more immortal and powerful than Love and Hate. In the Hall of Fame of immortal adversaries, they're up there with Good and Evil, Light and Dark, Yankees and Red Sox…
The speaker seems a lot more tentative about his relationship with hate. He was absolutely certain that he has tasted desire, but he only "thinks" he knows "enough" about hate. We're in murkier territory with this emotion.
For example, we don't know in what sense he knows hate. Has he only witnessed hate at a distance, or has he been the victim of hate, or has he been a hater himself?
His very hesitancy with the topic (what's he trying to hide?) should set off alarm bells of reader suspicion. Like most people, he has probably experienced several varieties of hate.
Why is hate an "icy" emotion?
You might think that hate should be red-hot because it is so closely related to blind anger. But we think he isn't talking about the kind of anger that comes on you in a fit of rage. We think he's talking about the anger that lingers beneath the surface, that you turn over and over in your mind.
You've probably heard the phrase, "Revenge is a dish best served cold." The idea is that, when you take what was originally a hot emotion and allow yourself to reflect on it in a very deliberate manner, you can come up with the cruelest response of all. Not to mention, the worst villains in movies are always the ones who kill and destroy without breaking a sweat or even seeming to care.
The phrase "also great" doesn't tell us anything about the status of ice compared to fire. So be careful about saying that ice is just as powerful as fire. He doesn't tell us that. When it comes to destruction, ice is in the same league as fire, but we don't know who would win a one-on-one cage match.
And would suffice.
The last line picks up on the idea that we don't know whether fire or ice is stronger.
The end of the poem suggests that it doesn't matter which is stronger: both will do the trick. "Suffice" means to be enough to bring about some effect.
Once you destroy something, it's gone. If you wanted to destroy some document, setting it on fire would do the job. So would tearing it up into a million little pieces. But if you set the document on fire and then wanted to scatter the ashes to the corners of the earth just to be sure, someone would be justified in saying, "Hey, what are you doing? It's over."
Interestingly, the word "suffice" is a very rational and even lawyer-ly word. The speaker seems to be demonstrating "icy" thinking here. If you got a little chill up your spine at the end of the poem, that might be why.