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 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.