If our ancestors had reflexively clung to the handful of names bequeathed to them by their forbears, we would still be going around calling one another Hrothgar or Mogthra, Bringer of Sludge.
Sure, Grendel has an anger management problem, but then he is stuck with appallingly noisy neighbors, egged on by a beer-bellied Dionysus, King Hrothgar, the demon's own dad (Anthony Hopkins, whose paunch is one 3-D effect too many).