After Grendel's fight with Beowulf, in which Beowulf tears off Grendel's arm,
Then he went off beaten,/mankind's enemy, deprived of joy,/to find his death place.
From Beowulf and the Geats' perspective, Grendel's death is well deserved and an appropriate end for a monster at war with man and God, but from a mother's perspective, she has lost her only child. Human and animal mothers alike defend their offspring against danger and enemies, and we can reasonably expect that Grendel's mother will do the same:
And his mother, still/greedy and gallows-minded wished to walk/a sorrowful journey, to avenge her son's death. (ll. 1276-1278)
When the Beowulf poet recounts Grendel's mother's revenge, he does so with no tone of surprise. In other words, the mother's attack on Heorot should be an expected consequence of Grendel's death. Had the Geats known Grendel's mother was nearby, they would have taken steps to protect themselves by killing her before they celebrated.
In Scandinavian and Anglo-Saxon societies, if a man is killed, his killer is expected to pay wer-gild ("man-money") to the man's family--that is, a payment to compensate the family for their loss. If a man is killed in war, the man's friends are expected to take their revenge on his killers if wer-gild is not acceptable or possible. Anthropologists often call this a cultural imperative--that is, if certain things happen, your culture expects you do take certain actions. Grendel's mother's revenge, then, has two justifications: 1) as a mother, she is taking revenge on those who harmed her son; and 2) even though she is a monster herself (although probably a descendant of humans), her revenge is consistent with actions that Beowulf and his men would take if one of their number were killed in an attack.