I just don't think
Elrond would have seen any good reason to intervene--certainly not to the point of killing or injuring
Isildur. Isildur was at that point an ally or friend, and--as far as I can see--displayed (at that point) no signs of "turning" or ambitions of setting himself up as another
Dark Lord.
Given the letter that
Gandalf reads thousands of years later, it is quite possible that Isildur didn't quite grasp
the Ring's power or significance when he took it from Sauron--perhaps only
the Wise had that knowledge. Thousands of years later, Gandalf would not force
Frodo or
Bilbo to relinquish the Ring: they would have to give it up of their own accord. There is, perhaps, an
evil fate for those who would take the Ring from another by force: they either perish (e.g.
Gollum,
Boromir), or become
Sauron themselves. Gandalf understood that: surely Elrond would have, also.