which scene was it when Leo really started to fall apart?

Hmmm, good question.

Leo had a legit reason to be upset with Bob and there were HUGE boundary issues however when Leo let this feeling of righteus grievance go to the point of acting immature was when he had the issue with his son diving and was so upset that Bob helped him that he shoves his son in. Also when the are eating he tells Bob that he can call him Leo in his office but he has to call him Dr. Marvin in his home. The first signs of him starting to actuall act "crazy" seemed to come as soon as the Good Morning America interview didn't go the way he wanted it. It was all downhill after that.

When he pushes Bob into the lake after seeing Siggy take his first dive.

I agree the GMA interview was the final straw. Up until then, he was annoyed but begrudgingly put up with Bob for his family's sake... and expected him to leave first thing in the morning. Could also be when even his doctor friend/colleague and everybody else was charmed by Bob, and nobody was sympathetic to his side or just forgot about him, lol.

Don't get me wrong, Dr. Marvin was a huge self-absorbed d*uchebag who seemed out of touch and no fun to be around....but I felt a bit sorry for him in this regard. Jerk or not, he still had the right to go on vacation and be left alone. Bob was an easygoing decent guy but just wasn't taking the hint.

Frankly I think the craziness goes back to before the events of Bob. When Leo first sees Bob and talks to him he tells Bob he never gets angry or upset. This is what leads to his insanity later. He suppresses his emotions so much that he goes insane later when he doesn't get his way to the point where he tries to kill Bob.

Green Goblin is great! https://www.youtube.com/watch?v=t1L4ZuaVvaw

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After the interview, when he yelled at Bob and kicked him out of the house.