One of Spider-Man's biggest symbols is the Green Goblin himself. Now, we know what you're thinking: "hold up, a character can be a symbol?"
The Green Goblin symbolizes Spider-Man's double identity; both men lead double lives. While the supercharged Green Goblin is ordinary businessman Norman Osborn by day, Spider-Man is totally average Peter Parker when he's out of costume.
Gobby is a mirror image of Spider-Man, albeit a dark and twisted one. They're total opposites, right down to their costumes, which are complimentary colors.
Spider-Man is noble like his Uncle Ben and strives to use his superpowers to help mankind, whether that means stopping a robbery, saving a baby from a burning building, or slapping the Green Goblin around. The Green Goblin, on the other hand, uses his enhanced abilities to terrorize those he feels have wronged him, like Gen. Slocum, Balkan, Fargas, and Spider-Man. He's as selfish as Peter is generous and willing to sacrifice himself for others.
Uncle Ben famously tells Peter that "with great power comes great responsibility." The Green Goblin represents what would happen if Peter had an anvil dropped on his head, forgot those words, and ran wild through the streets of New York City sowing superpowered chaos.