At this point in the story, Victor has recanted his promise to the monster. The monster has promised that he will leave Victor (and all of mankind) alone if Victor will create a companion for him, someone hideous like himself that will understand him. Victor attempts to do so, but cannot make himself finish and ends up destroying what he has done.
The monster knows what has happened. He barges in on Victor and says
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!
He uses the word “slave” to show that he believes he has power over Victor, and that he will be able to inflict revenge on him. He leaves him with the foreshadowing line
I will be with you on your wedding-night.
The monster considers this appropriate revenge. He plans to kill Victor’s bride as retribution.