Surely after she and Peter ultimately decided to be together and go to Oxford and before helping Spider-Man defeat Electro and before getting killed by Harry Osborn as director Marc Webb's chosen cinematic Green Goblin, Gwen had a choice that would have ensured that she would still be alive when Spidey webbed her hand to a police car, but she refused to stay put and got herself into the crossfire of Spider-Man's battles with Electro and Green Goblin and she was completely in over her head not listening to Peter as well as telling him not to make her decisions for her, even though it was worth helping Spidey successfully defeating Electro and restoring power to the city just before the Airplanes in the sky were about to crash into each other.
So now she's dead and that is what she got for disobeying Peter's suggestion of not getting involved.
Surely the reason Harry murdered Gwen was when he deduced Spider-Man as Peter Parker over him being with Gwen alone in the grid with no one else around, and because of his desperation on finding a cure to his disease as well as Spider-Man not giving him his blood drove him completely insane, he felt betrayed and wanted Peter to understand how it feels for hope to be taken away so dearly.
Clearly Peter couldn't risk giving him his blood even though nothing has happened yet because he wasn't sure if it was truely safe since the Lizard events in the Amazing Spider-Man 1. But surely Peter himself didn't know what Harry would do or how he would react after rejecting the blood transfer.
Well now it would seem the real reason why Marc Webb denied Mary Jane from appearing in the film and wanted Gwen to be seen killed off was all just to show how you can't always have a safe and perfect life when being a hero and an average citizen. But between you and me, I'm glad she's dead and done because of lame it is to have the name of an actress playing a not super power girlfriend put inbetween the names of the two actors playing the hero and the villain of the movie and because of Emma Stone's version of the character being stubborn, arrogant, hot headed, and completely disobeying Spider-Man's suggeting to get out of a dangerous area and not get involved in his fights, she's been a pain in the ass if you know what I mean.
But even though Gwen's death was really really sad, the movie ended okay because of Peter coming back as Spider Man to take down the Rhino.
Hopefully Mary Jane will be smart to listen to Peter once she learns he's Spider-Man and not end up like Gwen and Black Cat will be a more reliable help of a love interest to Spider-Man as well.