Nebuchadnezzar was not a good guy, by Biblical standards. He was also unpleasant company for a few years when he came to believe he was a cow and ate grass. What caused this madness? It might have been how he chose to decorate his palace.
King Nebuchadnezzar, in history, overthrew Jerusalem, put out the reigning king's eyes, and brought many Jews to Babylon as slaves. In the Bible he was even less popular, erecting a graven image, and throwing three people into a furnace when they refused to worship it. The three emerged unscathed. Nebuchadnezzar, on the other hand, "was driven from men and did eat grass as oxen." The first known case of boanthropy (thinking oneself a cow) did not clear up for seven years. Officially, he was being "humbled" by God.
Some historical experts have a different idea. It seems that at one point the historical Nebuchadnezzar decided to redecorate his palace. He had the walls painted a beautiful bright yellow, with a paint that would have included both lead and antimony. Neither is a chemical anyone would want to mess with. Antimony can give people seizures, inflammation of the heart, headaches, and depression. Lead is more of a problem, though. Lead poisoning can make people paranoid, delusional, and aggressive. The madness associated with it has been blamed for the fall of Rome. It's also been posited as a source of inspiration for the genius painters in history. Lead poisoning was at one point known as painter's madness - although it affected house painters as well as great artists.
The exposure to lead paint, everywhere, may have been a factor in a kind of madness overtaking Nebuchadnezzar. It provided a kind of immortality to him - the painting above is a depiction of Nebuchadnezzar by William Blake - but was fatal to his dignity.