In his last soliloquy of Act IV, Scene 4, having observed that Fortinbras is ready to risk his life "for an eggshell," Hamlet recriminates himself for his own inaction. While he feels that it is right to not stir oneself to action without "great argument," or deliberation, he also feels that it is wrong "to find quarrel in a straw"; in other words, to engage in debate where none is necessary. Hamlet, then, becomes active and seeks revenge,
....O! From this time forth
My thoughts be bloody, or be nothing worth!
I am not sure exactly what you are referring to, but Hamlet considers himself a coward. He listens to his father, but he is worried. He does not do anything to avenge his father's death or protect the kingdom. He regrets this, and becomes a little unstable. Basically, he acuses himself of not doing enough to stop the king.