Like her son, Grendel's mother is a descendent of Cain on the warpath for revenge. However, while Grendel primarily resented the Danes for their warm camaraderie, and the fact that, as humans, they have God's blessing, Grendel's mother hates the Danes because they hurt her son. Indeed, it is not until Grendel returns mortally wounded from his battle with Beowulf that Grendel's mother leaves the swamp and seeks her revenge.
Unlike her son, however, Grendel's mother receives comparatively little attention in the poem: she wreaks havoc in Herot, and then is swiftly dispatched by Beowulf. Even so, it's worth noting that, monstrous as she may be, she actually acts in very human-like way. Like a human mother, she is motivated primarily by the need to protect her offspring. As such, while Grendel's mother might be characterized as a villain in the poem, its impossible not to feel some semblance of pity for her, just as one would feel for any human mother who had recently lost her only son and companion.
Grendel's mother is as fierce as her son and seems even more vindictive. She is determined to get revenge for her son's death, thus exhibiting a strong maternal instinct. She too, is a descendent of Cain, the first murderer mentioned in the Bible. Thus, she is immediately connected with evil. She, like her son, are no match for the Danes who must call upon Beowulf for help against her attacks.