How did Victor's personality change while he was working on the monster?
I think the easiest way to answer this question would be to focus on Victor's interactions with other people. After all, our personalities are often displayed more obviously when we are around others. How does Victor interact with his family members before he begins to work on his creation? How good is he about keeping in touch? How much time/thought does he devote to Elizabeth? What about Clerval, his best friend? How much time does he make for Clerval, and what are their interactions like? Once he begins work on the monster, which of these relationships change, and how? The changes you see in the relationships will reflect changes in Victor's personality. Plus, each change will serve as a piece of evidence to support your ultimate answer to the question.
Victor, the scientist who creates the monster in Mary Shelley's book "Frankenstein" is a man who enjoys learning. He goes on a quest for knowledge in creating life. He is a kind and loving man. He has a bright future awaiting him. He is jovial and fresh when he begins his work. He dreams of being able to create the perfect being. His mother's death has affected his need to create a healthy human being. He wants to be able to end disease.
The being is created but instead of the success that Victor had hoped for, he finds that he has created a sinister being. He has brought upon the world an evil that he knows is his fault.
As the story progress Victor undergoes a dramatic transition. His time becomes allotted only for his work. He shuts himself away in his laboratory and does little else. He forgoes communication with family and friends. He is obsessed and driven by the evil that he has brought into his life and weighed down by the knowledge that he has created this problem. He no longer looks forward to his future and instead begins to regress physically as well as mentally.
In the end of the story Victor dies a peaceful death. He has done what he could to make things right and put the cycle of life back in order.