Why Does Grendel Attack Herot
What causes Grendel to attack the mead-hall in Beowulf?
Grendel and his mother are purported to be monstrous demons and the descendants of the biblical Cain, who murdered his brother, Abel, out of jealousy. Cain was punished by God and left to wander the earth forever in suffering with no way of alleviating his pain. The literal reason Grendel has for attacking Herot is that he hears the songs of King Hrothgar's bard, which tell of God's creation of the world, and the sounds all of the hall's inhabitants engaging in their joyous feast. Grendel despises these people for their happiness because, through circumstances beyond his control, he has been condemned to a life of suffering and despair in the shadows, so, much like his forefather, he is driven by his jealousy to commit violent acts against those whom he sees as favored by God.
Grendel is the spawn of Cain, purely wretched by nature. His entire being is devoid of joy. Some think that when he hears the jubilant parties that take place at Herot he is filled with intense jealousy because he has never experienced anything happy. This jealousy may also be interpreted as hatred, which seems to present Grendel as pure evil, a rather one-sided view of the story. Some scholars prefer to present Grendel as a creature filled with bloodlust, but presenting him as a sad being without love experiencing envy seems to present a more complex story with more realistic emotions.
Yes, as an agent of evil Grendel does attack Herot because he hates/envies the happiness and prayers of the warriors who reside there. One additional comment to make is that Hrothgar and his people are shocked not by just Grendel's carnage and bloodlust but also because there is no way to make amends with him. There is nothing they can do to stop him. His motive are not clear. There is no price they can pay to stop the murders (note how Hrothgar once helped buy peace between Beowulf's father and rival people). This makes the attacks that much more difficult for Hrothgar and his people.
Grendel attacks because he is evil (spawn of Cain) and hates the happiness and noise of the men at the hall. He attacks killing 30 men, and then goes back the next night for more. The only way men can stay alive is to leave the hall.