Elrond, why?

[Angry Hill Troll]

Elrond says the following at the Council (FOTR II, Ch2):

It should have been cast then into Orodruin's fire nigh at hand where it was made. But few marked what Isildur did. He alone stood by his father in that last mortal contest; and by Gil-Galad only Círdan stood, and I. But Isildur would not listen to our counsel.

This scene, it seems to me, was actually portrayed in the movie quite faithfully to the book.

Probably, at the time, the extent of the Ring's corrupting influence was not clearly understood. It was through knowledge of its effect on Isuldur, and later Sméagol, that Gandalf was able to understand its true nature.