The speaker, in some ways, sounds mildly incredulous. Sometimes when you're trying to convince yourself of something, you have to say it more than once. That's one of the bigger sound markers in this poem.
But there's also a kind of understated quietness about it – there's no grandiose language in the second half of the poem, even though it's describing a man who is about to kill tens of thousands of people. Instead, we have a scene that, while shot through with a kind of low rage, is utterly quiet: a man feeds sweets to a parrot. While contemplating his dead mother. And genocide.