Look, I'm not asking for a huge exposition on how he turns, but something a little more than what was shown.
Here's what should have happen:
After the musical when Harry left blew off Peter, we go back to Pete and MJ. After that scene, we should have cut to Harry's Mansion. There in his father's lab, Harry is getting drunk and just plain lost. He's conflicted about his feeling and doesn't know how to handle the situation. Then in a drunken stupor, he either accidentally hits something or spills his bottle that triggers the Goblin Gas to go off. He freaking out, and Norman's voice comes back, stronger, completing the psychotic breakdown. Then, Harry mind becomes more clear and realizes his father's mission and understands the need to take his revenge.
Or something to that effect. Nothing more that a 5 mins scene at max.