It's a book with multiple interpretations, like any halfway-good bit of art. Only absolute schlock has moral clarity.
The monster obviously isn't the good guy, as he strangles children.
The baizou thing of "the monster was good" makes no sense. Take any other murderous incel or child-killer and apply the same. Most people who do heinous murders didn't have easy lives prior to that; it's not a justification.
But the monster gets to give his side of the story a lot, in long monologues. I feel some people took them monologues too literally, said, "This is the message of the book", and I took it as the distorted ravings of monstrous psychology, with his subjective validity.
Yeah, but I think given Shelley's circles more radical tendencies, the interpretation of some revolutionary allegory is a strong one, especially when you think of that Rousseau(?) poem about the ruling class creating the 'monsters' that will destroy them.
The Monster isn't some child killer with a tough past, he's a child born into an adults body, cast out into the cold by his creator, and then spurned on account of his perceived inhumanity by every living being.
Most murderers get accepted by some initially, and when they don't it's on account of their bad vibes. The Monster showed himself to be very emotionally capable in spite of his troubles, and capable of living amongst humanity, especially in his covert benevolence towards the blind mans family. He even rescues a child, but is then shot at because people perceive him to be a monster.
I think it would've been fairly easy for readers at the time to say "the monster's a murderer, that's the story." It's easy for others to say the monster's a victim and not responsible for his actions. We see the two points of view throughout this comments. The comparison to an incel is almost silly, though. "Man decides women are The Problem and becomes dangerous" is different from "Man is abandoned and feared by his only parent at birth, he has the impulse control of a small child (recall he was literally just born) and he has the body of a very large adult man." He even does try to be good, but is angered by people dehumanizing him for his appearance. (Dehumanizing people based on things they have no control over: sound familiar?) But then there's the important bit: this does not absolve his crimes! He is, in fact, a murderer. But the people who hated him also bear responsibility. Most of all his parent who did no rearing, teaching, or literally anything but screaming and running bears responsibility. Is he a killer? Yes! Is he a victim? Yes! Is Victor responsible for everything? Also, yes! The people who rejected him responsible? Less so, but yes!