Brynjolfur, Bjoergolfur's son from his first marriage, then takes over the farm. He doesn't want Hildiridur and her sons around so he drives them away and refuses to give them any of Bjoergolfur's inheritance.
Then, when Brynjolfur dies, his son Bardur takes over the farm and also refuses Hildiridur and her sons of inheritance. Bardur then gets killed but bequeaths Thorolfur Kveld-Ulfsson his belongings. Hildiridur's sons don't like that so they go to Thorolfur to get a share of the inheritance but do not get anything.
They want to disrepute Thorolfur so they try to get the king to distrust him or even try to kill him. They went through all this trouble just to get a hold of Thorolfur's inheritance because they felt it belonged to them.
I don't think it was right of them to persecute Thorolfur because he didn't do anything to them. If they really wanted to get their share then they should have persecuted Brynjolfur or Bardur because they were related to them, Thorolfur had nothing to do with it. I also believe that their accusations against Thorolfur just describe how greedy of power they were. They inherited a good land from their grandfather on their mother's side, and had enough money but it wasn't enough. They wanted more. All th is greed got them to the grave because they were killed by Thorolfur's relative as a revenge for Thorolfur's death.