This is an interesting question with multiple answers.
According to the text, Grendel attacks the mead-hall after nightfall, when its inhabitants are asleep, snatches thirty or so of them, and then feasts upon them in his home.
I think it's likely that Grendel has no other motive: he attacks for food alone. Lots of predators that we are familiar with are night-going (cats, snakes, wolves, and owls, to name a few), because that is when their prey is asleep and open to attack.
With so much prey sleeping in one place, such as Heorot, it would be silly for Grendel to not take advantage of the opportunity and attack, as a man-eating predator.
There's also a 70's novel called Grendel that tells the story from Grendel's point of view. In this, he attacks while the scops are singing because they tell the story of Cain, the story of where Grendel's ancestry came from. He has no sympathy for the humans he eats because he observes how hypocritical they are, and how cruel they are to each other.
Personally I think the whole Cain thing is a insert by the monk who wrote it down in an attempt to Christianize the story, but the interpretation is still valid if you choose to take faithfully what was written down.
Grendel hates Heorot because he is envious of the joy and celebration that takes place there. He is also tormented by the sounds of music and laughter that remind him of his loneliness and exclusion from human society. Additionally, Grendel is angered by the boastful behavior of the warriors in Heorot, which fuels his feelings of resentment and bitterness.
It seems as though Grendel was excessively envious of the people laughing and celebrating in the great hall of Heorot. His jealousy overwhelmed him and he would attack and even devour some of the people inside.
grendel is hungry, I think
the disturbance created by the people
Grendel hates the noise of feasting and celebration, the sounds of joy and laughter, and the happiness of the warriors in Heorot hall. These remind him of his own isolation and suffering.
Grendel hates Herot because he feels rejected and isolated from the celebrations and joy happening there. The noise and revelry of the mead hall remind Grendel of his own loneliness and he sees the people in Herot as a threat to his existence. Grendel's violent and chaotic nature also fuels his hatred towards the hall and its inhabitants.
Grendel terrorized Heorot by launching nightly attacks on the warriors there, killing and feasting on them. His presence instilled fear and chaos among the people, making Heorot a place of suffering and despair.
Beowulf and Grendel fought in the mead hall of Heorot, where Grendel had been terrorizing Hrothgar's warriors. The battle took place at night when Grendel attacked the hall, and Beowulf vowed to rid the hall of the monster.
The hall in which Beowulf battles Grendel is called Heorot.
The hall in which Beowulf battles Grendel is called Heorot.
Grendel harasses Hrothgar and the hall of Heorot for twelve years in the epic poem Beowulf.
Grendel haunts Heorot for twelve years before Beowulf arrives to help the Danes.
The battle between Grendel and Beowulf takes place in Heorot, the hall of the Danes.
Heorot is destroyed by Grendel's mother in retaliation for Beowulf killing her son. She breaks into the hall at night, kills Aeschere, and kidnaps Grendel's arm. Beowulf then seeks out and defeats her in her underwater lair.
Grendel attacks Heorot, the great hall where the people are fighting, killing many men.
Grendel attacks the kingdom ruled by King Hrothgar, known as Heorot. He terrorizes and kills the warriors in the kingdom, causing fear and chaos until Beowulf arrives to confront him.