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.