From chapter 10 of the Project Gutenberg ebook - or Volume 2, Chapter 3, Page 102 of Penguin Classics edition of Frankenstein (emphasis mine):
My rage was without bounds; I sprang on him, impelled by all the feelings which can arm one being against the existence of another.
He easily eluded me and said,
“Be calm! I entreat you to hear me before you give vent to your hatred on my devoted head. Have I not suffered enough, that you seek to increase my misery? Life, although it may only be an accumulation of anguish, is dear to me, and I will defend it. Remember, thou hast made me more powerful than thyself; my height is superior to thine, my joints more supple. But I will not be tempted to set myself in opposition to thee. I am thy creature, and I will be even mild and docile to my natural lord and king if thou wilt also perform thy part, the which thou owest me. Oh, Frankenstein, be not equitable to every other and trample upon me alone, to whom thy justice, and even thy clemency and affection, is most due. Remember that I am thy creature; I ought to be thy Adam, but I am rather the fallen angel, whom thou drivest from joy for no misdeed. Everywhere I see bliss, from which I alone am irrevocably excluded. I was benevolent and good; misery made me a fiend. Make me happy, and I shall again be virtuous.”
The monster calls Victor his "natural lord." What exactly does this title mean?