Why did Hamlet reproach himself in his fifth soliloquy?

In his fifth soliloquy, Hamlet learns from the captain of the Norwegian army that the Norwegian prince young Fortinbras is leading his army against Poland to fight for an inconsequential plot of land:

"We go to gain a little patch of ground
That hath in it no profit but the name.
To pay five ducats, five, I would not farm it;
Nor will it yield to Norway or the Pole
A ranker rate, should it be sold in fee."

Hamlet, once he is alone feels guilty and begins to flagellate himself in his fifth soliloquy. He likens himself to a beast which does nothing else but eat and sleep without making use of the faculty of 'reason' given to him by God. He states by thinking and planning over precisely he has been reduced to a state of inaction and cowardice. He remarks that he is ashamed of himself when he witnesses this army which is setting out to do battle by risking the lives twenty thousand soldiers for an "egg-shell." He who has to avenge his father's murder and mother's shame is surprised at his own inaction and cowardice. He concludes by saying that from now on being inspired by young Fortinbras' example he will be more determined to kill Claudius:

"O, from this time forth,
My thoughts be bloody, or be nothing worth!"