Be warned, I discuss events in Amazing Spider-Man #598, so if you haven't read it...
So in the recent issue of Amazing, Norman states his big plans for his son. Harry is to work alongside him as the American Son. And if Harry screws up...well, Norman will have him killed.
...huh?
I know, I know. Norman's the resident EEEEVIILLL bastard over at Marvel now. But I still don't think Norman was ever that bad that he'd kill his own son. Belittle him, maybe. Try to make him into a 'man' worthy of his father? Definitely. But murder? No. I still think that, on some level, Norman still cares for his son. Anyone remember the Ultimate Green Goblin? It's a different universe, sure, but when he unwittingly killed Harry over there, he was so emotionally devastated that he effectively committed suicide when he found out what he did.
Now, let's turn the clock all the way back to exactly 500 issues ago with Amazing Spider-Man #98. Norman has just regained his memory of being the Green Goblin, and attacks Peter, who is with Harry. Harry is recovering from a drug overdose.
Anyway, later in the issue, when the two are fighting again (thanks to the Goblin's weapon, Spider-Man's not able to crawl walls)...
And, later in #121 (everyone should know what happens here)...
And we all know what happens here, so I'll stop for now. But its clear that Norman's anguish over Harry's illness is one of the leading factors that causes his mind to snap and bring the Goblin persona back.