Why does the monster call Victor a "slave" in Chapter 20 of Frankenstein?
In Mary Shelley's novel Frankenstein (chapter 20), the monster declares Victor to be his slave.
“Slave, I before reasoned with you, but you have proved yourself unworthy of my condescension. Remember that I have power; you believe yourself miserable, but I can make you so wretched that the light of day will be hateful to you. You are my creator, but I am your master;—obey!”
Prior to this chapter, the monster divulged to Victor his "coming of age," or how he came to be, after his "birth." After the telling of his story, the monster asks but one thing of Victor: for Victor to create a mate for him.
Chapters later, Victor upholds his promise. He begins to make a mate for the monster. In the middle of his task, the monster appears at the window of the hut where Victor is completing the mate. Victor is overwrought with emotion.
I thought with a sensation of madness on my promise of creating another like to him, and trembling with passion, tore to pieces the thing on which I was engaged.
By destroying the mate, Victor breaks his promise to the monster. The monster, enraged at the destruction of "his future existence he depended for happiness," fled "with a howl of devilish despair and revenge."
Later, the monster returns and calls Victor his slave. This naming of Victor as his slave shows the inevitable power the monster will retain over Victor and his life. Victor knows that the monster will seek revenge for the destruction of his mate. Given that Victor will spend the rest of his life in fear, the monster has become the master (the one with power) and Victor has become the slave (the one without power).
The creature called Frankenstein a slave since the creature believed that he holds power over him. In a chapter before 20, the creature threatened to kill Victor's loved ones if Victor refuses to fulfil his wish of making him a female companion. As a result, Victor went against his own will and began creating another creature in order to keep his loved ones unharmed, and thus the creature believed Frankenstein is his slave.