The first two were fine, the third crossed the invisible line of destroying the actual plot.
*Spoilers, but not really*
In the book, when Gollum tries to steal the ring from Frodo at the foot of Mt. Doom, the ring manifests itself into a ring of fire and speaks. It vows that if Gollum ever touches it again it will destroy him. Thus, later on when Gollum steals the ring from Frodo, the ring fulfills its vow by smiting him into the lava. The problem is that Gollum is still holding the ring when this happens and it destroys itself.
The point being, evil destroys itself. The movie opted for a bunch of gibberish and having Gollum just flip out and act like an idiot at this final part.