Even a parrot can roll an R! (lines 59-60)
With this quote, we learn that Trujillo thinks that even birds are more worthy of life than the Haitians, who cannot roll their "r"s in the way that Dominicans do. Thus, Trujillo views the Haitians as much less than human, which makes it easier for him to justify killing 20,000 people.
He will order many, this time, to be killed for a single, beautiful word. (lines 70-72)
Why? Because he can; because he has the power necessary to see his hatred through in this way. Oftentimes hate simply seethes – which is awful enough in and of itself, even when it doesn't end violently. But here hate does (or, rather, will) end tragically and we see it coming, and by the end of the poem we know that the cause is rooted in tremendous prejudice.