Why was Grendel so perturbed?

There are two examples of why Grendel was so perturbed in the epic poem Beowulf. First, examine Grendel's origin. He is a descendant of Cain, a Biblical figure who murdered his own brother in a fit of jealousy. According to Beowulf, God punished Cain by cursing his lineage, so that he would become a father of monsters. Grendel himself is monstrous, and in line with his heritage, he is an outcast, and "God did oppose him." When King Hrothgar builds Heorot, the Danes rejoice and celebrate, praising God for his blessings. Grendel, much like his ancestor Cain, attacks the first night after Heorot is built, in a fit of envy. The Danes' joy drives him into a rage, and he slaughters thirty men, returning each night to the mead hall in order to kill more people.
The second reason behind Grendel's perturbation is Beowulf himself. For twelve years, Grendel attacks the men at Heorot every night. He is unable to be killed due to a charm that makes him impervious to weapons, and the Danes live in fear of him. Beowulf comes to Heorot specifically because he heard about the monster Grendel. King Hrothgar is so relieved that someone has come to their aid that he declares, "Has in his hand, the hero-in-battle. / The holy Creator usward sent him." Hrothgar believes that Beowulf has been sent by God as direct opposition to Grendel. Grendel is once again facing God, in the form of Beowulf, who cursed him long before he was even born.