Elrond, why?

[Vanya]

Yes ,Simiel, if there was time enough for thinking.
The Ring never came in anybody's hands (man or elf) before Sauron was defeated. So, there was no experience on that subject (influence on man).
The Ring probably spoke to Isildur, and it was rush and bad decision which was result of ignorance, primarely. Luckily, he was slain by orcs, before it made real demage to him (turning him inti nazgul or something similar).
I don't think he could have been new Dark Lord, since he couldn't have had the power to wield it. I belive that Elrond sensed that, at least, even if he couldn't prdict the exact outcome

Isuldor was a powerful man, noble in sprit and mind. Elrond could not just say "Do as I say pathetic mortal", the whole thing was about choices, free will and not being subjected to the demands of another (for that was Sauron's desire to dominate all other living creatures).

Another consideration was that Elrond had some inkling of the power of the ring and thought that he may be unable to destroy it and therefore sought for a pure spirit to do the deed. I do not think the second option is viable, but then again, one never knows with pointy ears.

As others have said, Elrond probably had a moment of bad judgement. Yes, he should have made Isildur put the ring into the fire, or pushed Isildur in with it if he wouldn't surrender it. But give the guy a break! He did just fight a no-holds-barred contest with an immortal god. I'm sure he was quite tired after such a conflict and wasn't thinking clearly. Apparently Isildur wasn't thinking clearly either, as is evidenced by his subsequent repentence (see Unfinished Tales) and wasn't just stupid.

Besides Elrond being exhausted, how often have you had a gut feeling that something wasn't right, but went along with it anyway and it turns out that your gut was right? I get them all the time. Elves aren't omniscient (see e.g., the flight of the Noldor in the Silmarillion, they behave like asses), but they do act in a better way more generally than men do.

Another thing, don't believe the movie version. As entertaining as it was, it seemed to me that most of Mr. Jackson's rendering suffered from an excess of exhuberance. That is, in Mr. Jackson's version, it wasn't enough for Elrond to disagree with Isildur about the ring, but he had to SHOUT it at him because he knew that it was terrible and men were weak and stupid.