Does Grendel seem like the type to kill for fun?

Yes I would say he does kill for fun. Everybody kills in this story but Grendel seems to get some sort of satisfying rush from one of his many slaughters. All the music and "good times" in Hrothgar's mead-hall does seem to piss this demon monster off but his killings are largely fun filled blood splatter fests followed by a few snacks on the dead bodies.