Why did Victor Frankenstein reject his creation? Was it a failure on his behalf to fully realize what he was going to create? Was he "grossed out" by the end result? Was he supposed to be the prototype for irresponsible parenting? All are possible explanations for the actions of his character, but they assume too much. Victor Frankenstein is but a tool for Mary Shelley to use to make other comments and observations about real life regarding Man's limited ability to tolerate those who are different, Man's struggle to overpower nature and Man's tendency to have a "God complex".
Clearly, one of the main points that Shelley makes through this novel is that Man tends to reject and eventually hate that which differs from his own way of thought. The creation of Frankenstein is physically different, grotesquely so, from those he encounters. Only the blind man, De Lacey, showed any compassion towards the creature. His blindness made it so he could only judge the creature on the content of his character, which is fairly pure until it is jaded by the hatred society directs towards it. Frankenstein, and all others who encounter the creature, are appalled by his physical appearance and therefore judge him as evil and a monster. Historically, how many humans have been treated poorly for no other reason than their outward appearance? Millions. Shelley has, with the exception of De Lacey, all who encounter the creature reject him based on superficial assessment in order to show the destructiveness of that trend with human civilization.
Another reason for Victor rejects the creature concerns the struggle between Man and Nature. As Victor creates this being, he is attempting to rival the creative powers of Nature. In doing so, he creates a thing of no outward beauty. The reader may think that he is aghast at what he has done due to the grotesque result but it may be more appropriate to consider that he sees himself a failure compared to the workings of Nature.