^ My thoughts are whenever Peter loses someone he loves, which isn't as often as people think, it comes with, as expected, much sorry, but it fundamentally affects his character.
First we start with Uncle Ben, whose death started to teach him the "with great power comes great responsibility".
Then we have Gwen's death. In the comics this death added numerous things to his understanding.
First, that he wasn't going to be able to save everyone. Up until that point he had almost always been successful in saving those who were put in harms way. Sure not Uncle Ben, but he wasn't right there in order to attempt to save Ben. Yes his actions affected the situation, but that is still the "with great power comes great responsibility" thing. Even in ASM1 he was able to save the little boy from the burning car. Yes he didn't save Captain Stacey, but he didn't necessarily love him, although he did/does love the character who loved the Captain. You get the point...
Second, it showed him that there would be times where he had to choose the greater good. With Green Goblin giving him the choice of either the bus load of kids or the woman he loved he was put in a no win situation, and again, although he tried, he couldn't save both.
Thirdly, and most importantly, Gwen dies because of him, per se. Yes, it was Green Goblin that put him in that position, but it was his action of trying to save her with his webbing that actually caused her death ("swick"... "Snap"). In some ways he caused her death and that is something he had to live with for the rest of his life. The whole "what if". ("Don't be dead Gwen. I don't want you to be dead...")
So, does Gwen need to die because of Spidey trying to save her with his web? No, but I think it adds a lot to the story.
Also, does it have to be Green Goblin that causes Gwen's death? No, but my opinion is that so far the villains have not really been villains. For example, the Lizard was a good man under a bad influence. In the end he redeemed himself by "saving" Peter (Why couldn't Peter have just grabbed the side of the building when the Lizard's arm broke apart?). It appears (though we'll see) that Electro will be something similar, meaning he isn't being portrayed (I don't think) as maniacal. And even the Rhino doesn't seem Maniacle. Maybe brutish, maybe thiggish, but not a crazed psychopath.
That's why it needs to be the Green Goblin. His character takes things to a whole new level. He's really one of the first villains that the audience should hate. Again, did you hate the Lizard? Are you expecting Electro to be portrayed in a way where we will hate him? But Green Goblin, this time I think they'll do it right. This time instead of Norman being a "good" guy and just under the influence of his other personality, I'm guessing they will stay away from the bad multiple personality thing and just make Norman what he really is, a psychopath. Doesn't matter how he became that, whether through the serum or not, he is what he is, and that is someone willing to murder knowingly.
That's the real difference...
Looking at all of Spidey's villains there really are only a couple that fit this level. Green Goblin is definitely one, and I'd say Cletus Cassidy takes it even further. Venom came close, but he at least had some level of honor. And I think that that's the difference. Gwen's death can't be an accident, even one cause by a villain. It can't be a stray bolt from Electro that causes her to fall off the bridge. It has to be straight out murder and only a psycopath, one that the audience will want to see Spidey get revenge on, can do this.
I think that is why it has to be GG. He's the only villain that we are expecting in ASM2 that matches this tpe of character...
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