Why is Grendel angry in the beginning of Beowulf?

Which monster? There were three. I believe that Grendel was angry because the people in the mead-hall were being all noisy and disturbing Grendel. Grendel's mom was angry because Beowulf killed Grendel and they put Grendel's arm on display. She attacked to avenge her son. Then the dragon was angry because some people stole a gold goblet from the dragon.