Ah, well....I don't really think that Fortinbras is more WILLING to avenge his father opposed to Hamlet. Hamlet just thinks things through way too much. I mean, he is trying to be absolutely certain it was his uncle who killed his father.
The entirety of Hamlet pretending to be mad, is to draw out any conclusions to his Unlce being the killer. And the preformance of the play within the play
They are both, completely willing, but go about it in different ways.
Just kind of what I remember from the play. I don't know if it will help, but it could be something to take in.