What obstacles and motivations does Grendel have in Beowulf, and how does he overcome them?

Grendel was annoyed by the building of Herot, King Hrothgar's hall of his soldiers. Once the hall is built, the soldiers move in and begin celebrating. One night the noise of the celebration awakens Grendel and he comes to Herot and kills several men. He keeps killing Hrothgar's soldiers until they move out of Herot--thus restoring peace and quiet to Grendal's land. However, when Beowulf arrives, he already has a reputation as a brave fighter and monster killer. The first encounter Beowulf has with Grendel occurs just after the monster has eaten one of Beowulf's men. Then Beowulf wrestles with Grendel. Grendel was unaware of Beowulf's strength and cannot break his grip. The only way he can escape is to rip away and leave his arm in Beowulf's grasp. However, this causes a lot of bleeding to Grendel and Grendel flees back to his lair where he dies from loss of blood.

In Beowulf, what are Grendel's intentions and what impedes him?

Grendel primarily wants the noise coming from Hrothgar's hall to stop.  He is bothered by the cacophony and its implications of men having a good time (he is miserable in his watery lair.)  

 There is not much preventing Grendel from achieving his goal until Beowulf shows up.  He snarks down warriors like they were Cheetos.  But when Beowful does come to rescue the kingdom, all bets are off.  The terrifying Grendel has met his match in ferocity.  He will be defeated by the great warrior, and his arm will hang above the hall (claw and all) as testament to Beowulf's prowess. 

 I believe Grendel is really undecided on what he wants to do.  The poet tells us that he is a descendant of Cain who has been banished to the swamps and dark places by Lord for his murder of Abel.  Thus, I think that on one hand he is jealous and really wants to join the men of the world,  but can't since he has been placed where he is by the Lord who banished Cain.  Since he can't join them, he cannot stand the sound of the music and enjoyment that he hears each day so he wants to destroy them.  I believe he is limited in his ability to destroy because it seems he cannot go far from his lair and maybe cannot be out by day. Furthermore, he might not really want to destroy Hrothgar's people entirely, he may just want to keep them from enjoying the mead-hall at night.  In this he is successful for 12 years until Beowulf comes to Hrothgar's aid.

What motivates Grendel in Beowulf?

Part of the answer to this question lies in the allegorical nature of the text and the way that Beowulf presents a titanic struggle between the forces of good and evil, with evil of course being represented in the form of Grendel. Throughout the text, every reference to Grendel uses adjectives such as "fiend in hell" and "ghastly atrocities." Such descriptions clearly associate him with demonic, evil forces that are opposed to the good that is represented in the order that Hrothgar tries to sustain and Beowulf defends. Consider the way that Grendel is presented in the following description:

...misery of men; that damned creature,
grim and greedy, soon was ready,
savage and cruel, and from their rest seized
thirty thanes; thence back he went
pround in plunder to his home, faring
with banquet of bodies.

The text does not give any indication of a specific motivation that lies behind Grendel's attacks: there is no reference, for example, to a particular grudge that Grendel has against Hrothgar. It is enough that Hereot exists and that Hrothgar is there with men. Grendel's status as a "damned creature" who is both "grim and greedy" and "savage and cruel" indicates that his motivation for murder stems from his evil nature. As the personification of evil, he is bitterly opposed to the goodness presented in Hrothgar and the men at Hereot. As the force of evil, he is implacably opposed to any force of good and seeks to destroy Hrothgar's kingdom.

What does Grendel want in the poem Beowulf?

Grendel simply wants to be left alone.  He visits Herot because the noise from the celebrations and libations of the warriors is unbearable.  It hurts his ears.  Therefore, he goes there to seek revenge for the suffering they have forced upon him.  Of course, his revenge might be more deeply seated since he is said to be descended of Cain, the first murderer on earth.  As the Anglo-Saxons value loyalty to family above all else and Cain killed his own brother, Cain would be considered guilty of the worst possible crime.  As a descendent, Grendel would have lived a miserable life.  Grendel may blame all mankind for this rejection and life as an outcast.  Their joy is in direct contrast to his misery and suffering, and therefore warrants his wrath.