In The Fall Of The House Of Usher, Roderick and Madeline enter a pub on New Year's Eve. They get talking to the bar woman, who eventually reveals
she knows that they have murdered their boss and arranged to take over the company. At this point, she makes a deal that she will ensure they get away with it in exchange for the lives of their children.
That's fine for Madeline - she doesn't have any - but Roderick already has one by this stage, and we find out he has five others by the time the payment is due. It seems he is so greedy, he is willing to sacrifice them for success.
However, by this point he and Madeline are portrayed as already having felt like they had the situation under control, that they were confident that they would get away with it and that their plan was working.
So why the need for such a drastic sacrifice? Verna (the barwoman) doesn't seem to be threatening to go to the police or anything - the exchange is relatively amicable. Are we supposed to think that she is subtly threatening to reveal all if they refuse? But the scene doesn't really play out like one of blackmail.