This is an opinion question, naturally. However, I think a huge case can be made for Victor being the more monstrous. One can compare him to a parent who grossly neglects his child. When the monster is first created, he looks upon him with disgust and abandons him, leaving him to flounder in the world by himself. The creature's first tendencies are toward kindness as shown by his treatment of the DeLacey family and his saving of the young girl. It is only when he kills William that his angry and evil tendencies show. He is like that chlld who cannot get attention from his parents for being good, so he chooses to get negative attention. All of the creatures actions stem from Victor's neglect, making him the more responsible and the more monstrous.