Back to the
Let's skip to The Green Goblin part of the story. Norman Osborn is CEO of a huge technology company that currently has the contract to supply the US Army with futuristic super soldier technology. Unfortunately the army aren't happy with just a futuristic 'glider' machine, they want the promised drug that will turn ordinary men into super soldiers. Although Norman tries hard to brush over this fact, it arises that not all of the animal tests have been successful -- one test caused the rats to become violent and insane. Human testing cannot go ahead, the project is set back months. On hearing this the Army decide to take their money elsewhere, to a competing company with a cybernetic exoskeleton.
Norman is a ruthless businessman, he's willing to take risks to stay in the game, so that night he goes to the lab after hours and he and one of the scientists (coincidentally the one who told the Army human testing wasn't safe) begin human testing of the drug, with Norman as the willing volunteer. Unsurprisingly the risk turns out to be real, Norman immediately becomes violent, brutally murdering the scientist whose ethical standards put his company in trouble.
Cut to the next morning, Norman is unconcious on his office floor. He doesn't remember what happened, but when he's told that his head scientist has been murdered and the glider has been stolen he reacts with shock and remembers that he was with the dead scientist the previous night. He gets flashes of memory... but what happened?
As the story goes on we see The Green Goblin, he rides around on the stolen glider, wearing a green suit and blows up people and objects that stand in the way of Norman's company OsCorp. The most obvious example of this is when the Army representatives, board of the rival company and their exoskeleton are destroyed in one sweeping attack.
Back to Norman, things are going well for him, he's reading in the paper about how OsCorp's got the Army contract afterall. But what's this? Is that the sound of evil laughter in the distance? Could this be the insanity that was foreshadowed earlier? Norman searches around the room for where the laughter is coming from. Now he's being called to. He asks who is this talking to him, gazing around the tribal masks hung on his wall. Eventually Norman finds who it is, it's his own reflection in the mirror! The Goblin is in the same body as Norman, while Norman's been blacking out the Goblin has been out killing off anyone who gets in Norman's way. Unsurprisingly Norman is shocked and scared by this revelation. He doesn't usually go around killing people and he's scared to find that he's been unknowingly sharing his body with a psychopath. Norman looks scared and pleading, in the mirror the Goblin has a twisted leering expression and speaks with a deeper voice. At other times Norman talks pleadingly to the Green Goblin's mask, begging him not to kill any more.
It seems to be the age old stereotype of multiples in drama, but then the tables start to turn. The OsCorp board of directors tell Norman that they've sold the company to its biggest competitor and part of the deal is that Norman is not included. It seems that he's going to lose everything afterall. Wouldn't it be convienient if something bad were to happen to the entire board?
From this moment on it seems that Norman and the Goblin are working together. As Spider-Man starts to fight The Goblin, it is Norman who shows suspicions as to who Spider-Man is and it is Norman who finds out Peter Parker's weakness from a conversation with his son. In the very final battle scene where the Goblin is killed, Norman apparently comes to the front to beg Peter to spare him and help him be free from the Goblin but this is infact a double cross, while Peter is distracted Norman (or perhaps the Goblin) is activiting the glider by remote control. It seems as if Norman has embraced the work of the Goblin and they are no working together cooperatively towards the common aim of revenge against the board and the defeat of Spider-Man (although this is not explicit, in the later part of the film we are not shown Norman and the Goblin talking any longer but we are shown Norman being more agressive and having suspicions about Peter Parker without any evidence of the Goblin coming to the fore).
In conclusion, while this seems to be a bad multiples in film stereotype, I don't think it's really that simple. Norman Osborne still deserves the (terminal) fate of the Green Goblin because he's clearly shown going along with the psychopathic behaviour of his bodymate at the end of the film. Even though at the start it's a genuine surprise to him that the Goblin had killed people. Once he realises that the Goblin is making all his problems 'go away', he starts to go along with atrocities. Instead of a stereotypical 'you could be a serial killer and no know it' plot, it's actually quite a sophisticated portrayal -- even though Norman 'lost time' to start with, once he realises the truth he doesn't try to stop it and he eventually goes along with it. People in multiple systems have a shared responsibility as a single legal entity and an ethical obligation to stop others from causing harm if they are aware of it. Norman covers up the Goblin's crimes and eventually actively participates in them.
On the otherhand we're still left with a film where the villain commits crimes from the body of an unknowing host and where multiplicity is portrayed as scary and implied to be insanity, and all just for cheap dramatic effect. It could be seen that the multiplicity is thrown in as a B-plot to give more body and suspense to the middle part of the film and, once this dry patch in the plot is passed, the Goblin becomes more of a clean cut villain. It's really up to the viewer to decide.