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.