[The Saucepan Man]

Although this is in the Movies forum, it is worth bearing in mind that the scene in the film was an invention of the film-makers. In the book, they were not at the Crack of Doom but (I think) on the slopes of Mount Doom. So, even had he been so inclined (and I agree with those who have said that he would not for the reasons that they have stated), it wouldn't have been a matter of simply pushing Isildur into the Fire. He would have had to have attacked him. Bearing in mind that they would have been surrounded by their respective armies, that would have been a very unwise move on Elrond's part. Isildur's soldiers were unlikely to have stood around and watched Elrond pummel him to the ground. And the Elvish warriors would not have taken kindly to their intervention. Battle would undoubtedly have ensued. So much for the Last Alliance ...

Another point to bear in mind is that Elrond was not immune to the Ring's effects. Both Galadriel and Gandalf were tempted by it. Elrond would have had little chance of withstanding it there at Mount Doom where it was most powerful, let alone destroying it (Tolkien makes clear that no one could willingly have destroyed it, Bombadil excepted). So, even had Elrond successfully "overcome" his nobility and killed Isildur, and then escaped the ensuing carnage, the Ring was likely to have prevented him from destroying it, even had he attempted to carry Isildur's body to Sammath Naur and drop it in.