This question poses a very subjective answer. What this means is that any person can justify why Victor (from Shelley's novel Frankenstein) rejected his "child."
Victor realizes his mistake the moment he looks into the eyes of his monster.
How can I describe my emotions at this catastrophe, or how to delineate the wretch whom with such infinite pains and care I had endeavored to form?
The instant Victor sees his monster, he realizes that his obsession with re-animating life was not one which he completely thought through. Victor is appalled at the monster.
Perhaps the reason Victor rejects his creation is linked to the inhuman life before him. It is as if Victor has had the ultimate "ah ha" moment.
Victor seems mostly disturbed by the eye looking upon him. What one could assume from this is goes back to the old saying that the "eye is the window to the soul." Victor did not like what he saw when looking at the soul of the monster; or, Victor failed to see a soul when looking into the eye of the monster. Either way, Victor suddenly realized his mistake.