I feel a lot of sympathy for the creature. If he is ugly and scary, it is because Frankenstein did not do a good enough job making him look human. Also, the monster's behavior is a direct result of Victor's inhumane treatment of him and turning his back on him.

In Chapter 17, Frankenstein's creature tells him,

I am malicious becaue I am miserable.  Am I not shunned and hated by all mankind?  You, my creator, would tear me to pieces and triumph; remember that, and tell me why I should pity man more than he pities me?

What, then, is "monstrous" are the actions of Victor Frankenstein which have initiated the malice in his creature. In those chapters in which the creature speaks of his acquaintance with the Delaceys, Chapter 12 and 13, his first response to humans is benevolent and he has a strong emotional response to nature.  His loving and unselfish attitude toward the Delaceys reveals him as more human than Victor. 

Frankenstein's creature might have a monstrous appearance, but that doesn't necessarily make him a monster. I think during Shelley's day he would have been considered a monster. Certainly, she intended him to be an unnatural creation. Modern readers aren't likely to view the supposed monster in the same light. Personally, I think he started out as innocent as a child. He eventually commits some monsterous acts, but he was certainly provoked. The questions is whether or not provocation excuses the action. He is certainly not guiltless, but I do not think he is a monster. Generally, I think someone is a monster when they can do terrible things and not feel guilt or remorse. Frankenstein's creation does feel guilt and remorse. Perhaps the real monster is Frankenstein himself.

I, too, do not look at Victor's "son" as a monster. The creature was created as a result of Victor's obsession with reanimating life. The creature was not an "accident." Therefore, the creature only reacted against the actions taken against him (people attacking him based only upon his appearance and Victor's alienation of him). One could easily suggest that if the creature were embraced and loved that he would not have murdered. For me, if I were constructing an essay on the topic, I would take the stand that the creature was not a monster based upon his experience in life. Underneath, the creature was seen possessing very humanistic characteristics based upon his actions taken with the De Laceys.

I think it's unfair to call the monster a monster.  It is actually less monstrous in its actions than many "regular" people.  It only starts to go bad and kill people after it is pretty dreadfully provoked.  Lots of people kill for less reason than the "monster" had.

Shelley's "creature" is not the sort of "monster" we are used to from the films starring Boris Karloff.  For example, he is highly intelligent and extremely articulate; in those respects, he might as well be a professor at Oxford or Cambridge.