In Beowulf, why does Grendel become angry with Hrothgar and the men in the hall?

Although Grendel attacks Hrothgar's mead-hall, Heorot, when he hears the laughter and celebrations of Hrothgar's men, the motive for Grendel's attack, and the origin of his hatred for men, is complex.

We must look to Grendel's origin and place among men to understand his hatred. The Beowulf poet, for example, introduces Grendel as

That fiend from Hell,/that grim spirit. . . . With fulsome monsters/this sorrowful man had stayed awhile,/Since the Shaper [that is, God]/had condemned him as Cain's kinsman (ll. 101-107)

As part of the Christian overlay of the poem, Grendel is linked to the slaying of Abel by Cain, and as a descendant of the Christian religion's first murderer, Grendel has been banished from the world of men. More importantly, Grendel is completely aware that he cannot join in the celebrations of mankind.

Grendel's anger and hatred of men are less about control of territory and more about envy and revenge:

That sinful creature,/grim and greedy, was instantly ready,/savage and spiteful, and seized at rest/thirty thanes. . . . (ll. 120-123)

The killing of Hrothgar's warriors, in modern terms, could be called "collateral damage" in the sense that Grendel's hatred is created by his outcast state rather than simple hatred of mankind--he hates mankind, of course, but his hate derives from his inability to share mankind's joys. When Grendel is described as spiteful, the Beowulf poet is reminding us that Grendel is outside the circle of men and Christianity, and Grendel's anger stems from his isolation from the Christian world.

Grendel's control of Heorot is not an end itself, then, but a by-product of his sense of isolation--even though he controls the hall, his feeling of isolation from God is paramount:

Heorot was his,/the treasure-decked hall . . . (but he couldn't approach the throne of God/for the Lord's gift, nor know His mercy) (ll. 166-169)

One of the Beowulf poet's goals in this poem about an essentially pagan world is to create Christian elements, and one of the most powerful of those elements is mankind's relationship with God. The Grendel episode gives the poet the opportunity to create a character (Grendel) who is both a monster and an object of pity--he behaves montrously, and is condemned for that behavior, but from a Christian viewpoint, Grendel is also to be pitied because his isolation from God and men derives from his heritage, something he cannot control.

The noise from the building of the great hall, Herot, as well as the noise generated from the many parties held there irritated Grendel who was used to having control over this area...so Grendel did what every neighbor does when you're angry about too much noise; tear their limbs off.