What is the climax in Frankenstein?
The climax of the novel is when the monster murders Elizabeth on her wedding night. Tension has been building toward this moment: the monster has requested a mate of his own, and when that request was finally denied, he vowed to Victor, "'I shall be with you on your wedding night.'" It may seem fairly obvious to readers that the monster plans to kill Victor's bride and deny Victor her love, just as Victor has denied him the opportunity to be loved by another. However, Victor assumes that the monster only wishes to sign his own "death-warrant," and fails to recognize that the threat is against his beloved. The monster has never sought to kill Victor, only to force him into the same agony of solitude that the monster has had to endure. Thus, the dramatic irony created when we understand more than he does helps to build tension leading up to the fatal night as well.
On the day of the wedding, Elizabeth "was melancholy, and a presentiment of evil pervaded her." She seems to have some sense, some premonition, that something terrible is going to happen, but Victor continues to complain about his own suffering, and he says that only she can relieve it. The scene in which all of the built-up tension is released is when Victor hears Elizabeth's scream, finds her body, passes out, and awakens to see "A grin [...] on the face of the monster; he seemed to jeer, as with his fiendish finger he pointed towards the corpse of [Elizabeth]." The moment readers could predict has finally come, and Victor has been rendered almost completely alone in the world, his one chance at love ruined forever by his own narcissism and ambition.