Once again we are treated to a special-effects-fest in which there is an insidious temptation to let the effects gloss over weaknesses in the plot rather than exist only because they are justified by the plot. The temptation is not always successfully resisted. So we have plotholes, like, if the Black Goo can survive a meteor impact (!), how come it doesn't survive the bomb at the end? But I actually thought "Spiderman III" was more interesting than II (though not as good as I) because it tried to explore the dark side of human nature, and hence the plot did matter; whereas II was just about special effects.
That is not to say it completely succeeded. Peter Parker is taken over by a symbiote (the Black Goo) from outer space which amplifies the aggressive side of human nature, causing him to act selfishly and seek vengeance rather than justice. It envelopes him and takes the form of a black Spiderman suit (as opposed to the normal red and blue one.) The imagery is imaginatively effective, but theologically troubling, for evil is depicted as essentially external. Peter has to struggle against this evil influence. But he is perfectly capable of controlling the evil from within all by himself until this alien influence comes along. And even then it is his own innate goodness that finally overcomes it. He is aided by the resonance of the bell (which the Black Goo apparently doesn't like), but though that scene takes place in a church, the aid is a purely natural phenomenon. It is not supernatural grace.
The key moment in the struggle is when Peter is in the bell tower trying to "take off" his black spidey suit. I immediately thought of Eustace trying to take off his dragon skin in Voyage of the Dawn Treader. What a contrast! Peter pulls off (ahem) his transformation back to goodness all by himself, aided only by the resonance of the bell, while Eustace has to give up his own efforts altogether and let Aslan undress him, which involves a tearing down to the very center of his soul. The difference is profound. It is the difference between secular wishful thinking about human nature and the radical biblical truth of the Gospel of Christ.
I'm glad I saw "Spiderman III" because it makes the perfect foil for C. S. Lewis's treatment and shows how brilliant Lewis's imagery actually is. One appreciates the effort, but secular people finally have a very superficial view of what evil is and what it takes to overcome it. At least the Spidey writers did understand that Peter's forgiveness of Sandman has to be the end result. They got that part right. Who knows what evil lurks in the heart of man? Not the "Spiderman" movie writers, apparently. But at least they tried. We can pray they are on the way.