WMG / Beowulf

Grendel's mom wasn't hostile to humans until Grendel was killed.

Notice how whenever partygoers are being maimed and eaten, it's always Grendel himself doing it; mommy dearest seems content to stay in her damp cave and let humans be. It's only when her child dies in her arms that she goes on a rampage. It may even be that in the lost original pre-Jesus-Hijack version of the legend, it wasn't that Grendel's kind (whatever they were originally supposed to be) were Always Chaotic Evil, but merely that Grendel in particular was just a massive jerk.