Grendel kills his own people because he enjoys striking fear into the heart of the innocent. No matter how much you you defend yourself, if somebody kills another, you’re the monster.
Grendel's' mother only wants to kill somebody because, Beowulf defeated Grendel and let him bleed to death. Beowulf finally left to the hills while trying to find the sanctuary. Eventually Beowulf finds the Dragon. It only went on a fiery rampage just because one piece of the mound of wealth it slept lazily on was taken. In other words the dragon killed out of greed. Again, Beowulf strides into the picture with his companion Wiglaf. After the epic battle, they brought down the dragon. But not without a price. Beowulf suffered a mortal wound which put him to rest. These things to everyone else, are monsters and Beowulf is visualized as a hero. Grendel kills for fun; his mother for revenge; and the dragon kills for greed. Finally, Beowulf kills for fame and honor. The only thing that seems to be separating the hero from the monsters is their reasons for killing and that's it.