Why does Victor Frankenstein change his mind about making the monster?

Because his creature is monstrous in appearance. Victor also regrets tampering with nature and the natural order of things once he witnesses the anomaly that his monster has proven to be. However the reader can later perceive that though monstrous in appearance the creature is really sensitive, intelligent and vulnerably human. The creature also kills numerous members of Victor's family either directly or indirectly in his quest for vengeance against the creator who abandoned him and condemned him to a life of loneliness and suffering, thus fueling Victor's hatred for him.