Lucius knew exactly why she chose to correct him. It wasn't a matter of family pride, as it would look from the outside eye. The reality was that Rodolphus was in trouble with the Dark Lord. if he didn't succeed in this test he had been given, he would be killed. He was going to be killed anyway, if it was going to be Bellatrix herself, or the Dark lord, no one was certain. by given the words she had spoken. The blood was to be on her hands. She had more then aggression directly towards him. The similarity in both Lucius and Rodulphus" behaviours drove Bellatrix to a murderous mind regularly.