Lord Elronds Mistake

[olorin hamfast]

Something occured to me whilst I was watching the New Line Version of LotR about a statement Lord Elrond made about the "Hearts of men", and how he saw the hearts of men fail;
" I was there that day"
He was with Isildur in the Chasm at Orodruin after the King had fallen and Isildur had cut the ring from the the Decievers hand.
"Cast it into the fire!" yelled Elrond
"No" and Isildur took the ring for himself, causing all sorts of problems for generations to come.
Why did Elrond not finish the job then by sticking a couple of elvish arrows into the greedy bastsard's back and kick Isildurs Bane into the fire for once and for all? Did he not fail the hearts of the Last Alliance? Im sure Mr Tolkien did not intend this to happen, as it wouldve cut short his royalties, but Lord Elrond wouldve properly served the efforts of the Last Alliance and put a quick and dirty end to the whole third age. Also, would we expect Samwise to the same to Mr Frodo when his moment of truth came that day in the bowels of the mountain of fire?!?