What is immediately established by the contrast between the goat and Grendel?

Grendel's encounter with the goat in Chapter 10 follows a scene in which he was processing the life-and-death philosophies that the Ork shared with him. These ideas have caused some misgivings about his approach to life.
As the goat climbs up, Grendel takes note of the goat's persistence but he does not welcome it. Rather than admire its tenacity, Grendel is annoyed and throws stones at it. He keeps stoning the goat until he splits its skull, and even when it seems the goat has died, still he does not stop.
The main contrast drawn is between passivity and aggression. The goat is not interested in harming Grendel, but it seems dedicated merely to making its way up the hill. Grendel, however, seems unable to resist attacking the goat.