In Beowulf they say Grendal is a monster because he is a decendnt of Cain. With little sicentific theroy of certn traits of people could be possable that Grendal was human. Since they never actally saw him?

Oh, bruther!

Grendel could be human. So the bad traits can be passed to descendants. I haven't read the book yet. But I am familiar with the story of Beowulf.

"- Tolkien
In 1936, J.R.R. Tolkien's Beowulf: The Monsters and the Critics discussed Grendel and the dragon in Beowulf. This essay was the first work of scholarship in which Anglo-Saxon literature was seriously examined for its literary merits—not just scholarship about the origins of the English language as was popular in the 19th century.

- Debate over description:
During the following decades, the exact description of Grendel would become a source of debate for scholars. Indeed, because his exact appearance is never directly described in Old English by the original Beowulf poet, part of the debate revolves around what is known, namely his descent from the biblical Cain (who was the first murderer in Abrahamic religions).

- Monster:
Some scholars have linked Grendel's descent from Cain to the monsters and giants of The Cain Tradition.
Seamus Heaney, in his translation of Beowulf, writes in lines 1351–1355 that Grendel is vaguely human in shape, though much larger:
... the other, warped
in the shape of a man, moves beyond the pale
bigger than any man, an unnatural birth
called Grendel by the country people
in former days.

Heaney's translation of lines 1637–1639 also notes that his disembodied head is so large that it takes four men to transport it. Furthermore, in lines 983–89, when Grendel's torn arm is inspected, Heaney describes it as being covered in impenetrable scales and horny growths:
Every nail, claw-scale and spur, every spike
and welt on the hand of that heathen brute
was like barbed steel. Everybody said
there was no honed iron hard enough
to pierce him through, no time proofed blade
that could cut his brutal blood caked claw
Peter Dickinson (1979) argued that seeing as the considered distinction between man and beast at the time the poem was written was simply man's bipedalism, the given description of Grendel being man-like does not necessarily imply that Grendel is meant to be humanoid, going as far as stating that Grendel could easily have been a bipedal dragon.

- Non-monster:
Other scholars such as Kuhn (1979) have questioned a monstrous description, stating:
There are five disputed instances of āglǣca [three of which are in Beowulf] 649, 1269, 1512...In the first...the referent can be either Beowulf or Grendel. If the poet and his audience felt the word to have two meanings, 'monster,' and 'hero,' the ambiguity would be troublesome; but if by āglǣca they understood a 'fighter,' the ambiguity would be of little consequence, for battle was destined for both Beowulf and Grendel and both were fierce fighters (216–7).
O'Keefe has suggested that Grendel resembles a Berserker, because of numerous associations that seem to point to this possibility.

John Grigsby, in his Beowulf and Grendel :The Truth behind England's oldest legend' suggests that Grendel is a demonized version of the old Norse fertility god Freyr, and even goes as far as linking Grendel with the Green Knight of Arthurian legend."
Source and further information:
http://en.wikipedia.org/wiki/Grendel

Further information:
http://en.wikipedia.org/wiki/Beowulf