'Alas! yes,’ said Elrond. 'Isildur took it, as should not have been. 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 I will have as weregild for my father, and my brother,” he said; and therefore whether we would or no, he took it to treasure it. But soon he was betrayed by it to his death; and so it is named in the North Isildur’s Bane. Yet death maybe was better than what else might have befallen him.

Both knew the power of evil and could guess the amount of suffering caused by not destroying the ring. Why didn't they take it from Isildur by force? Even if they had killed him, it would be a small price to pay for saving thousands. It would even be an ethical decision towards Isildur. It was clear that he is getting corrupted in the very moment he decided to keep it (or perhaps even fight to keep it), and it was clear that he would suffer a lot from its influence and die in sorrow and pain - as Elrond himself says "Yet death maybe was better than what else might have befallen him."