[Kalimac]

Hmmm, hard one to untangle. The only chance Boromir really had to take the Ring was when he quarreled with Frodo at Parth Galen, and it's hard not to remember that at this time he was (a) near a river and (b) unbeknownst to him, on the verge of being attacked by a very surly group of Uruk-Hai. For some reason the first idea that comes to mind is that it might have been a repeat of Isildur's fate; Boromir would have had the Ring only briefly - long enough to attract the attention of the Nazgul et al but probably not long enough to have them actually come to him before the Uruk-Hai attacked. The Uruk-Hai attack and, intoxicated by the Ring, he decides to take a pass on defending Merry and Pippin and instead leaps into the Anduin to escape while the Fellowship is being attacked, the Ring slips off his finger, and...continue from there. Of course, this time Sauron would have known exactly where the Ring was and it would have taken him all of a few minutes to collect it unless the remaining members of the Fellowship did some very swift search-and-recovery work. Possibly Boromir could have used the Ring to command the Orcs, but it doesn't seem likely somehow. The Orcs don't listen much, and Boromir also wouldn't really know how to use it like that with so little experience.

Undoubtedly Frodo would have had to be dead or nearly dead to let Boromir get the Ring, but I doubt Sam would have killed Boromir for revenge simply because he never would have had the chance before the Fellowship was attacked; everyone was split up and confused by that point. And it's hard to picture Boromir trying to defend the hobbits after he's let the Ring get under his skin.