Victor Frankenstein's weaknesses include:
1. his over-vaulting ambition. In creating his monster and his hideous birth he hurt those around him and isolated him from the ones he loved.
2. Stubbornness. Victor was stubborn and unrelenting when the monster agreed to leave the world of man if only he had a similar mate to share his existence with.
3. His inability to assume responsibility in caring for and nurturing his creation. Things might not have turned out so horribly for Victor if only he had showered some love and attention on his creation. Instead, Victor was responsible for things turning out the way they did. If one was accepted and not abandoned, then the murders of Justine and Victor's brother wouldn't have happened.