The whole argument of him being out of his mind disregards one element of his disease: "it comes and goes."
Was Harry so out of his mind, that even after he had killed Gwen and got his revenge from Peter ("You took away my hope...now I'm going to take away yours!"), after the dust had settled and he was sitting with Fiers at the end of the film, that he's now going to form an entire group of vigilantes dedicated to killing him? Really? That shouldn't strike as a weak basis of wanting to kill someone to only a minority of fans.
Also, he was just so out of his mind that he couldn't put two and two together and realize that his best friend was trying to save his life, yet he learned, in a matter of minutes, how to ride his glider and make his way to the power plant?
Even when I went to see TASM2 with my cousin, my aunt (a very average movie goer) specifically pointed this out. I've also read this complaint on a countless number of posts, reviews, and websites. That should be a major concern, in my opinion. That shows that either a movie isn't doing a good job of explaining something, or perhaps it's far too elaborate for some people to understand. In this case, I think it's the former.
Norman points out to Harry that he had so much "potential" in the beginning of the film. That causes the audience to assume that Harry is probably an intelligent individual who has done little to benefit from his intelligence, or made the wrong choices in his life altogether. Given that, he seemed capable enough to understand that his best friend, supposedly a genius at science (I would probably take his advice, no?) is warning him of the possible drawbacks of attaining Spidey's blood. Additionally, as I said "two and two together" above, he didn't realize he had more time to live, considering his own father seemed to live a pretty decent length of time?
But no, he was so devastated by the fact that his best friend was actually a superhero that was trying to save his life, that he not only killed the love of that friend's life, but he's now going to form a team to kill him, too.
If I'm trying to look at it from Harry's perspective, it really isn't that difficult to grasp - especially if you're observing the situation prior to his transformation to Goblin. There wasn't anything that justified him taking such abrupt steps and completely disregarding the opinion of the guy we're supposed to believe is all he had, and has been his best bud from the time they were kids.
If you try to analyze Harry rationally, the only conclusion you arrive at is that the character was just dumb from the start.
