[Imladris]

Going from what Nuru said (jolly good post btw) is that Elrond would have committed murder by killing Isildur. I suppose that you could take the argument and say that by not murdering Isildur he was condemning thousands of people to death and that the fate of one was worth the fate of many. But think of it this way: Nobody's lives were in immediate danger from Isildur taking the Ring. Sauron was presumed soundly beaten and I don't think that many people really realized that there was a lot of danger in keeping the Ring. I think that it was a hind site 20/20 in the Councel of Elrond, where everything fell into place. But that is just my opinion from the general tone of Gandalf and Elrond in Shadow of the Past and the Councel of Elrond chapters respectively.

Taking the Ring from Isildur by force also would have caused considerable damage from what Gandalf mentioned in FotR.

If Elrond had killed Isildur it would have been bad form, as Hook would say.