"Here's how it really went Peter...blah blah blah...I didn't mean to kill your uncle."
And Peter believes everything he says. How naive. How does he know that Sandman wasn't lying so he would let him go? Wait a second...who would you believe? The cops that say Flint Marko, a man with a bad track record, aggressively shot down your uncle in an attempt to steal his car? Or Flint Marko who says he really didn't want to kill him, and he's sorry...bo hoo.
And Harry does the same thing. Bernie tells Harry that the blade from his glider killed him, and all of a sudden he's helping Peter. If Harry was insane as he was suppose to be, and was hellbent of avenging his father, wouldn't he have thought "then Spider-Man stabed him with his own glider!"
...
1: Why should Sandman lie in that situation??? He's standing there with ****ing tears in his eyes, when he could've continued on trying to smash Peter's head in. It wouldn't make sense for him to lie right there. And why would he stick around to explain that heartbreaking story to Peter, when he could've just blown away with the wind at any time, letting Venom keep on knocking the **** out of Peter. "I didn't want this, but I had no choice". Peter is standing there thinking he's a nut, replying "We always have a choice... You had a choice when you killed my uncle!". Then Flint lays it all on him, explaining what happened, and all he has left is his daughter. If he TRUELY wanted to kill spidey, he would've continued what he started to begin with.
However, had he really wanted to send the message home, he should've helped Peter when he was on the verge of getting the final blow by Venom (like Darth Vader seeing the evilness in Palpatine, and helping Luke in the nick of time.)
2: Harry wasn't insane like his father was! The performence-enhancers had side-effects to surden people, like aggression and insanity (like Dr. Strome says in spidey 1). In Norman's case it developed a form of split-personality, hence insanity... But they also had successful tests with the perfomence-enhancers, it just wasn't perfect. It all comes down to who can take the medicin, and who can't. Norman obviously couldn't! Harry could handle it! He didn't go completely insane, he just got the power of the medicin. Harry is under the impression that Peter killed his father, and the only problem Harry has, which was there way BEFORE he ever took the performence-enhancers, is the tormenting image of his father inside his head driving him nuts, ultimately making him give in to the anger. As the symbiote clouds Peter's judgement, so does the medicin and his fathers ghost (constantly inside his head) do to Harry. It doesn't necessarily make him completely out of his mind crazy and uncontrolable, like with Norman.
You also have to remember that the performence-enhancers wears off eventually, which is why there's so many of them in the goblin-lair. And the 2'nd time around, Harry doesn't wanna kill Peter any more, he just wanna make him suffer. Peter also told Harry "I didn't kill your father, he was trying to kill me, he killed him self!". In the end he connects that piece of information, to what Bernard tells him about the glider-piece found inside his fathers wound. You can also clearly see, that when Peter comes to ask Harry for help, Harry is totally calm, and is just sad. Besides, Harry knew from the start that his father was the Green Goblin, a complete and utter PSYCHO! He was in denial for a long time. So even if Peter stabbed Norman with his glider, Harry KNOWS deep inside of him self, that his father was the baddie, and Peter is a hero. After the rematch between him and Peter, Harry has had a long time to finally come to his senses.