Isildur

[Mithadan]

Another point is that while Bilbo and Sam "possessed" the Ring, neither "claimed" it, at least while having knowledge of what it was. Sam "borrowed" it but resisted the temptation to claim it. Bilbo considered the Ring to be his own, but without knowledge of what it was or its power, and even so had difficulty relinquishing it. Even Frodo did not "claim" the Ring until he stood at the Cracks of Doom. In contrast, Isildur had, at least, some idea of what it was and by considering it "weregild," actually claimed it. While this is a weakness on the part of Isildur, it may have played into his refusal to destroy it. Having claimed the Ring, it was beyond Isildur's power to destroy it.

[William Cloud Hicklin]

Sort of a necrobump, but-

4) Having refused to attempt to use the Ring to save his men because it would hurt too much, Isildur is perfectly happy to accept that same pain when it lets him run away. Even once he has it on his finger, and "Men and Orcs gave way in fear", he makes no effort to turn it to the defence of his son and his men; he just flees.

I'm not sure I read it that way. It seems that there is a difference between invisibility- an automatic function of the Ring when donned by a Man (or Hobbit) - and not even an intentional design feature, because of course Sauron never anticipated anyone but himself ever wearing it. Anyway, put on Ring, disappear is NOT the same thing as actually "using" the Ring to exert Power. That is something neither of our hobbits, or Gollum, ever even tried, not having the strength or innate power to do so; and apparently Isildur, mighty a warrior as he was, could not either: in DGF he says explicitly that this is so when his son asked him to do it. It would have taken one of the Wise, or perhaps an Aragorn, actually to wield the thing.