In Beowulf, he is a flat character, but in Grendel, he is a round character.
In Beowulf, Grendel is nothing but a mere monster, brainless, with no actual motivation other than to cause pain to people. We don't know much about him apart from the fact that he likes to kill.
On the other hand, Grendel in Grendel is different - he has human qualities, we can clearly see what his motivation is, and he almost seems more human than actual humans.