What did Belgian gentleman want to know from Dorcas?

From the text:

“My good Dorcas, it is necessary that I should know every detail of that quarrel as fully as possible. Do not think that you are betraying your mistress’s secrets. Your mistress lies dead, and it is necessary that we should know all—if we are to avenge her. Nothing can bring her back to life, but we do hope, if there has been foul play, to bring the murderer to justice.”