Final answer:
William Wilberforce argued that the slave trade was evil, advocating for its abolition using moral and ethical reasons to transform societal views on slavery.
Step-by-step explanation:
The primary argument William Wilberforce was making for the abolition of the slave trade in his speech to the British Parliament in 1789 was that the slave trade was evil. Wilberforce and other abolitionists used moral arguments to advocate for the immediate elimination of slavery, emphasizing the atrocities committed and the ethical imperative to create a society with equality for all, regardless of color. The movement leveraged the religious fervor of the time, aiming to transform the national conscience and bring about a recognition of slavery as a sin, aligned with broader ideals of freedom and human rights that were becoming more prevalent.