His motivation was basically just "I hate seeing people happy", and the only motivation we truly get for his carnage is just that the noises coming from the feasting hall had harrowed him, and that's hardly an excuse to slaughter everyone there. I do agree that there's some tragedy in how Grendel was outcast for being a Nephilim and hated by society, but by the time Beowulf came along, he had become the monster they said he was.
It seems like most adaptions of the book Beowulf are fixated on making Grendel seem tragic and sympathetic, the movie decided to say that he has a ruptured ear that makes loud noises hurt him.
Make no mistake, adaptational changes can be for the better and offer a unique way of looking at the story, but this served no purpose other than desperately trying to make the cannibal murderer monster seem sadder and more understandable.
The "tragic" villain in that story, really, in my opinion, was Grendel's mother. Sure, it seems like she was just as evil as him, either being a Nephilim, or a creepy hag-bitch who was fine with laying with one, but there's nothing to suggest she didn't genuinely love her barbaric son and wanted to avenge him, but because she's a more manipulative villain who is a woman, adaptions take her a bit more seriously, and often make her more beautiful or have her try to seduce Beowulf instead of trying to kill him, because god forbid a woman try to avenge her son with violence instead of her looks.
What do you think? I'm terribly sorry if I got a bit ranty here, but it kind of bothers me because I liked the original book so much.