[Carannillion]

I think Aragorn would have been most capable of killing Frodo. Aragorn (son of Arathorn and all that pompous *** stuff) only knew Frodo as a courier; Frodo had not yet become an official ringbearer, and they were not exactly life-long friends... Gandalf had of course given himspecific instructions on making sure that the Hobbits reached Rivendell unharmed, but how exactly would you bring a wraith along on a hard journey with nine other wraiths hunting you? Tie it up?

As the ultimate goal was to get the Ring to Mt. Doom 'faster than possible', Aragorn would - as a last resort - have been able to kill Frodo.