Working alone in his little abandoned shack, Victor has all the time in the world to think.
And he suddenly realizes that the new monster will have free will. This complicates things.
Even if monster #1 agrees to be peaceful, monster #2 might be furiously angry at being made so hideous. She might hate monster #1. (He is ugly, after all.)
Mrs. Monster might very well go on a killing rampage, and then whose fault would that be? Victor's, that's whose.
AND what if they had monster babies? The thought is too terrible for Victor to even consider.
In the middle of his work, with the monster watching through the window, Victor destroys everything.
He thinks he's done a good thing, and maybe he has. But he's broken his promise to a murderous monster, which is not so good.
The monster vows to exact revenge on Victor, promising in a very scary and not at all sexy way to be with him on his wedding night.
Unfortunately, one of Victor's main flaws is his obsession with himself. He assumes that the monster intends to kill him on his wedding night, despite the fact that the monster has made a habit of killing people Victor loves.