First off, let me point out one thing: The question "who is the real monster?" is far too black-and-white, far too simple, for this book.
To be fair, it was kind of long and rambling. What we are basically getting is this scientist’s letter to his sister, recounting Victor Frankenstein’s tale, a huge chunk of which is the creature’s reminiscence. This wasn’t necessarily bad. It is also an interesting look at how one can gradually fall into evil (although thus far The Picture of Dorian Gray is doing a much better, and subtler, job at this).
For those few people living under rocks, the story recounts the life of Victor Frankenstein, whose early loss of his mother has driven him to find a way to bridge the gap between life and death. To this end he studies the old alchemical masters, whose names you can find in any Lovecraft story. By piecing together various body parts he does indeed create a sentient being; but he is so horrified by what he has done he has fled.
What follows next is the embodiment of the phrase “come back to bite you”. The book is a good exploration not only of sin but of what happens when we ignore or do not deal with our sin. Because Frankenstein fled from taking responsibility for his actions, the creature was left to wander on his own. He begins to learn by living in an abandoned part of a house next to a poor country family; but their reaction to his hideous countenance fills him with hatred. From there, the creature has his own fall.
That is the part people overlook. The creature had begun to learn right and wrong, and wound up choosing wrong. Other people’s actions can affect our own, but in the end we can still decide whether or not we take a particular action.
Another part that struck me was how the end showed two different reactions to remorse. While it took forever for Frankenstein to finally admit his responsibility for the thing he created, he fled his evil deeds, refused to commit them a second time, and showed true repentance in the end. The creature pursues Frankenstein for the purpose of murder and does not show any remorse or concern until Frankenstein is dead.
The question about “who is the monster” is inaccurate, because there is no actual monster. Instead, there are two flawed characters who both experience the pain of sin.
While the book is tedious at times, it winds up being a very compelling read.