Final answer:
The incident that may have changed Victor's life is related to doubts about Bess's past with Oldring.
Step-by-step explanation:
The incident that may have changed the course of Victor's life is not explicitly stated in the provided information. However, based on the given text, it can be inferred that the incident that had a significant impact on Victor's life is related to a person named Bess and Oldring. It is suggested that Victor had doubts, hopes, and jealous fancies about Bess's past with Oldring, and he wanted to know the truth to kill the past. This incident seems to have shaped Victor's future plans and emotions.