What annoys Grendel and leads to his attacks at the start of Beowulf, and what universal conflict fuels his war with the Danes?
Grendel is not only annoyed by the noise at Heorot; he's also bothered by the merriment and joy that occurs in the famous mead hall. Grendel is a descendent of Cain, the famed Biblical outcast who murdered his brother and was condemned to wander the earth. Thus, Grendel is cut off from larger communities, and so the sounds of celebration and joy (which we can also see as representations of community, friendship, and belonging) anger him and drive him to attack the hall. As such, the universal conflict can be seen as not only the continuation of a Biblical conflict, but also as the conflict between an outcast and a community. Indeed, Grendel can be seen as an archetypal version of the outcast who tries to punish the community for exiling him and refusing him friendship. Viewed in this light, Grendel almost becomes a sympathetic creature, although he's pretty much represented as a vicious brute in the poem. If you're interested in seeing an alternate version of Grendel, John Gardner's novel Grendel is a good place to start.
Grendel is annoyed by the noise of the building of Herot, King Hrothgar's home for his soldiers. Then, the noise of the soldiers celebrating in Herot wakes him up so he begins a series of attacks. Beowulf arrives to help Hrothgar because Hrothgar had ended a feud for Beowulf’s father, Edgetho,when it threatened to cause war between the Geats and the Wulfings (a Germanic tribe).
Grendel is regarded as the descendant of Cain, the first murderer whose story is told in the Old Testament, and the sword Beowulf uses to cut off the dead Grendel's head is decorated with depictions of the Old Testament’s giants who were destroyed by the flood. Thus, the question "Am I my brother's keeper?" is echoed in Beowulf's response to Grendel's attacks. Beowulf obviously feels indebted to Hrothgar and eagerly defends the king and his men against a murderer.Unlike Cain, who killed his brother, Beowulf defends his fellow man.