What evidence shows that grendal symbolizes evil and that the conflict is between good and evil?

The exact description of Grendel is never directly given in Old English by the original Beowulf poet, what is known, is his descent from the biblical Cain (who was the first murderer in the Bible). Grendel is referred to as a Sceadugenga - "shadow walker", "night goer") because the monster is repeatedly described as in the "shroud of darkness".

When Grendel attacks the mead hall he does not touch the throne for it is described as being protected by the power of God. Hrothgar and his people helpless against Grendel's attacks, abandon Heorot.

Beowulf is a young warrior (who's heroic exploits are described in the early part of the story) from Geatland, hears of Hrothgar's troubles and with his king's permission leaves his homeland to help Hrothgar.

Beowulf and his men spend the night in Heorot. Beowulf uses weapons because he fears that he will be killed by Grendel, just like all of his previous opponents. During the battle, Beowulf has been feigning sleep and leaps up to clench Grendel's hand. The two battle until it seems as though the hall might collapse. Beowulf's retainers draw their swords and rush to his aid, but their blades cannot pierce Grendel's skin. Finally, Beowulf tears Grendel's arm from his body at the shoulder and Grendel runs to his home in the marshes and slowly dies.

Later Beowulf becomes king of the Geats and is lauded as great.