[Nurumaiel]

I feel that even if you would murder for such a cause, Elrond would not. When Tolkien wrote LotR he made a definite point through the actions of his good characters that they were good. Murder is evil, for whatever reason it is done. It would be ridiculous to have a character who is honourable, wise, and good contradict all those things by being dishonourable, unwise, and wicked. Elrond would have to be mad to feel it was his obligation to act as Sauron would by murderering so Sauron wouldn't have to.

Frankly, I can't tell you why Elrond acted the way he did, for I could never see into his mind, but I can give so feasible opinions on when the screenwriters made him do what he did. As they're basing Elrond of Elrond from Tolkien's books it would be a blatant inaccuracy to have him commit murder so Isildur could be stopped.