Why is Grendel so angry and homicidal?

Grendel's first encounters with the world outside are both marvelous and disheartening, particularly as his first moment of weakness leads to an attack by a bull, followed by torment at the hands of the first humans with which Grendel interacts. From that point on, Grendel's search for personal meaning intertwines with his desire to torment the humans and Hrothgar in particular. Grendel's encounters with the dragon and with various human agents only result in further frustration for the monster. The people he wishes to join ostracize him, so he practices further self-isolation by making his presence among them terrifying.