The monster strangles William. He then places a necklace ti frame Justine. This represents the monsters drive for vengeance. He wants Victor to suffer.
The monster confront Victor on the mountain. He tells Victor his story and then asks for a partner. He says that Victor owes it to him because of his failures as a creator. Creation is the central theme of this scene.
Overcome by his doubts, Victor destroys his creation. He does not trust himself to control the creatures he is creating. His previous failures are shaping his actions. Ambition & Fallibility.