Who is the real monster in this book?

We believe it is Victor

Why? Because Victor and his hunt for knowledge led him to create the creature, and as soon as he created the creature he immediately didn't want it and neglected it. Also, Victor never provided any sort of fatherly parenting towards the creature, and never taught anything to the Creature. So the creature had to go along learning things on his own. The creature starting to kill a bunch of people, because the first things he experienced in his social life was being beaten and shot. Later, he wanted his creator to give him a mate, which Victor agreed to at first and then denied after some time.

Victor's guilt drove him mad

He knew he had created what he thought was a "monster", but he never accepted what he created for what it was, and assumed it was just a "monster", and he didn't do anything to stop any of the killings when he had the power to all along.

Victor was Selfish

As a child all he wanted was to learn, he had a thirst for knowledge, and it didn't matter if it was old or current knowledge. As soon as he had met Elizabeth, he knew he wanted to keep her for forever.