In this one, Electro is mad because.... Spider-man sorta didn't recognize him once. Harry/Goblin is made because he has a genetic illness that *should* not entirely take hold for a few decades yet but he wants a cure now and Spider-man is unsure donating his blood to Harry is a good idea.
What they're doing just doesn't quite fit, make sense, or really seem all that fluid on why it all has to involve Spider-man.
Electro was mad because he felt betrayed and abused by everyone. He was ignored, abused and used by OsCorp (who stole his work on the power system and also were responsible for his mutation), and then when he thought Spidey was trying to help him he gets shot at and thinks the one guy who was his friend has now also betrayed him.
Then he gets tortured by Dr Kafka on OsCorp's orders, furthering his hatred of them. When Harry lets him loose, it's with the promise of revenge.
And that's really what he wanted, total revenge on the whole world for how he was abused and thrown away by everybody. Spider-Man was just one guy he wanted to get back at, he wanted everyone. That's why he said he would be their new God at the end.
Harry was afraid of dying after seeing what happened to his father (understandable, really) and didn't want to wait decades when he thinks he has a cure NOW. When Spider-Man turns him down, he goes for the only other immediate cure he thinks he has: The Spider-Venom. It sort-of saves him but mutates him into a mentally unstable Goblin thing. The transformation warps his mind, and he now wants revenge on Spider-Man for denying him something that could have averted his current mutation. Finding out Spider-Man was Peter just pushed him further over the edge because now he knows the guy who he blames for his mutation was also his supposed friend.
The thing is, Electro's story doesn't have THAT much to so with Spidey as it does with the world in total and Harry's story needed a bit more depth added to it. But the story is all there.