I was sent to gaurd the brat girl is she suppose to die. I killed all the assassins and now I am waiting forever with nothing happening except her telling me to go back downstairs in case there are more.

Apparantly, after you have killed all the assassins, her grandad turns up to collect her. There is a 50/50 chance he is an imposter, so use detect evil to check. Return to William Reirrac for your reward.

If this sequence didn't happen, did you kill her real grandad by mistake? If not, it could be a bug and you will have to reload.