In the play Hamlet, why is Hamlet disgusted with his mother’s behavior?

Hamlet's mother, while his father was alive, was completely enamored with her husband the king.  Hamlet himself describes how "she would hang on him, as if increase of appetite had grown by what it fed on" (I.ii.143-144).  So after her husband dies, the "expected" reaction would be devastated grieving for a very long time.  If she was so in love with him, his death would have shattered her.  However, Hamlet is very upset because "yet, within a month-Let me not think on't-Frailty, thy name is woman!-A little month...married with my uncle, My father's brother...within a month...she married"  (I.ii.145-155).  He is disgusted because she, whose love was so strong for his father, with seeming no regard or mourning, quickly married his uncle.  First of all, it is an insult to his father, and secondly, it is his father's uncle, which Hamlet considers "incestuous" and vile.

He becomes even more disgusted when the ghost informs him that it was most likely his uncle that killed his father.  So not only did Gertrude marry his father's brother, but a man who was a murderer to boot.  His anger towards his mother flavors much of the mood of the play; because of it, he unleashes an angry tirade upon Ophelia about the fickle nature of all women, which contributes in part to her unfortunate turn to madness, and later he verbally assaults his mother after the play.