It shouldn't be the Giant - Grendel.
Through a god-view perspective, Duncan observed and discovered that the monster's skin was wrinkled like a hardened keratin layer, layered like scales, thicker than many leather armors, and without any special name tag.
However, Grendel is a name given by the natives, so perhaps it hasn't truly made a name for itself yet.
According to legend, the monster plagued Denmark for twelve years, only encountering Beowulf, who then slew it, a few decades later.
"But it can't be ruled out that it has already been born."
"Giants have a very long lifespan."
With this in mind, Duncan hesitated to act rashly.
It's not that Duncan couldn't defeat this giant monster, but rather if it really was a young Grendel, then it would have a powerful banshee mother behind it, as Beowulf's famous battle was killing two monsters.
With a god-view perspective, Duncan didn't mind observing a little longer to see if this monster had any other companions.
