Wealtheow represents mankind, light, and good. Grendel’s mother represents evil, darkness, and danger.
Wealtheow, Hrothgar's wife, is a gracious and graceful host. She is also a mother, and she also cares about her sons. However, she behaves in a queenlier manner because she is queenlier. She is above the earth, and Grendel’s mother is below it.
“Drink of this cup, my lord, giver of rings! Be you merry, oh magnanimous friend of men, and speak gentle words to the Geats, as men should do! Be glad with them and mindful of those gifts you've received from near and far. (ch 17)
Grende's mother is described as a "hag" and a monster. She is described as the wife of Cain, and she lives in the deep caves below. She is angry when her son is killed, and goes to avenge his death visciously.
That hag was in haste, wanting to flee with her life when the liegemen spotted her. However, she seized a single clansman firmly as she fled to the moors. (ch 19)
Sometimes it is hard to remember that Grendel's mother is also a mother. She does not even have a name, and the descriptions of her are never pretty, but she is driven by the same deep maternal desire.