He probably could have taken him, but Elrond did not know for sure that Isildur would become evil & rise to power. He would not have wished to slay him when there was still a reasonably good chance of Isildur never becoming evil. Besides, Isildur repented of what he had done & was actually on the way to Rivendell to seek council as to what to do with the Ring when he was ambushed, killed, & the Ring was lost (I am 90% sure it was Rivendell
). So that proves right there that Isildur still had plenty of time to hand the Ring over to another, though he might not have chosen to do that.