[Mithadan]

Isildur exemplifies a recurring theme in Tolkien's mythos. He is a hero that fails the last test. Other examples are Frodo himself, Boromir, Denethor (whose entire line's sole duty was to preserve the kingdom for the return of the king), arguably Thorin and others. Of these listed, only Frodo survived his failure and suffered for it.

There are counter-examples as well; those who do not fail the last test. Galadriel, Faramir, Aragorn, arguably Sam, Elendil and others.

These two types of heroes are in stark contrast to one another. I do not know that "weak" is the right term for Isildur. As someone commented above, criticizing Isildur for failing to destroy the Ring is blaming him for being unable to do what may have been impossible. Frodo and Isildur couldn't do it and Gandalf was afraid to even try.