Goblin

Casting is a toss up. I will go with better characterization through Pearlman.

The Goblin set everything into place for that terrible moment to occur, and therefore bears the responsibility for Gwen's death.  Peter bears the weight of wondering if somehow, despite the Goblin's murderous actions, he might not have saved her after all, if he hadn't tried catching her in just that way... We in the audience know that Peter's webline stopped her too suddenly and broke her neck, but Peter didn't hear the "snap" that we, the readers, knew took place. He still thought she was alive as he was pulling her back to the top of the bridge.  "Spider-powers, I love you!  Who else could have saved a falling girl from certain... Gwen?"  He can't ever be certain if he caused her death or not.  One thing we can all be sure of is who knocked her off the bridge, and that's the Goblin.