Black Goku is the evil and corrupt form of Zamasu, a god who, disgusted by humanity, takes possession of Goku's body. His goal is to erase all mortals from existence because they do not fit his standards of perfection.
Zamasu does not tolerate imperfection. He wants a universe without errors, without disorder, without humans. But the more he talks about purity, the more he reveals his contempt. He does not want to purify the world, he wants to punish it for not being as he would like it to be and develops the Zero Mortal Plan.
Zamasu in Japanese means "to be." His divine name reveals his arrogance, he wants to be the only one to exist.
Black Goku is a tribute to the dark versions of other characters in Japanese works such as Kamen Rider Black, a dark anti-hero from whom Toriyama took inspiration. (Also, Black Goku is, hands down, a much better construction of a dark Goku than Turles)
Toriyama and his staff wanted Masako Nozawa to dub both Goku and Black Goku, but with a colder, sharper, icy tone. The same voice that represents two souls: one smiles at life, the other despises it. For the viewer it has a strong impact. (It is no coincidence that the Black Goku saga is the most beautiful of Dragon Ball Super.)
Nietzsche divided the morality of masters from that of slaves. The morality of masters is born from self-affirmation, from strength, from creativity, from life. That of the slave, on the other hand, is born from envy, impotence and resentment.
The weak cannot say "I am strong", so he builds a morality in which the strong become the bad. Thus the meaning of good is reversed: what is weak, passive, submissive, becomes virtuous. In theory, Zamasu should be on the side of the strong, the lords, the gods. Instead, he is a slave.
Erich Fromm wrote: "When we move away from love, we move closer to destruction."
Zamasu is afraid of the freedom of mortals, of their chaos, of their vitality. That's why he hates them. Because he can't control them. He knows that no matter what he dictates, not everyone will accept him definitively.
It's very easy to point the finger at others, calling them monsters, even if sometimes it's true. Hell is other people, says Sartre, but how difficult is it to point the finger at ourselves and admit our mistakes, our failures, our weaknesses, our imperfections?
When was the last time we apologized for our wrongs?
We end up hating the other because they remind us of our limits and we end up spending a life of resentment, hatred and rancor. Towards our parents, society, the world... Maybe we hate others because we can't forgive ourselves. For not being strong enough. But is it worth it?