The biggest problem Grendel's mother's lack of language causes for her son, in Beowulf, is the isolation that he feels with no one to speak to or connect with.
Grendel's dam (mom) never is able to share with him her feelings or offer Grendel any verbal love or acceptance. In this respect, he feels isolated, completely alone. When he hears the voices of the men at the mead hall raised in laughter and celebration, jealousy consumes him, and all he wants to do his take out his rage on the unsuspecting inhabitants gathered there.
(When reading Grendel, a novel told from the creature's point of view, the reader comes to better understand the creature's agony of loneliness.)
Grendel's only connection in the world is to his dam, and when she dies, even though they have had a limited emotional connection, he is driven beyond reason, killing as many humans as possible.
Grendel's lack of connection with his mother, prohibits a sense of connection with or compassion for other creatures in the area. This is what drives him on his path of attempted annihilation of human beings that share the area with him.