Frankenstein's monster has issued a none-too-polite demand for his creator to make him a bride. After seeing what havoc his creation has wrought, however, Victor's not exactly enthusiastic about the prospect of unleashing yet another monster upon an unsuspecting world.
Still feeling guilty over the deaths of his brother William and the housekeeper Justine, who was wrongly executed for William's murder, Frankenstein doesn't want to cause more suffering by making a monster that could be more dangerous than his previous effort.
So right in front of the monster, Frankenstein destroys his half-finished creation, the monster that would have been his original monster's bride. As one can imagine, the monster's far from happy about seeing his potential mate destroyed like this. Having bullied Frankenstein into making him a mate, he keeps up his bullying, issuing the blood-curdling threat that he will be with Victor on his wedding night.
"Frankenstein's monster" is a common expression for a situation that gets out of control and goes beyond what was originally intended. And it's not difficult to see how this applies to Frankenstein himself.
When he began to create his monster, he had no idea what kind of chaos he was unleashing upon the world. But now, as he realizes just what kind of a situation he's got himself into, he finds himself in a classic "tail wags the dog" scenario where he's being bullied and pushed around by his own monster.