Lady Cary wanted to kill two birds with a single stone. That is why she used her cigarette. She knew she could smoke her cigarette and enjoy it and at the same time use it to dominate the slave. The mistress used him as a human ashtray as she smoked and she was able to kill two birds with a single stone. She loved the whole thing and did not mind doing it again.
Lady Cary craved a smoke and she had to get one. She used her cigarette to clear her mind but as she smoked, she knew it was time to admonish her boyfriend for all the wrongs he had done. She did not want to let the chance pass so she used that chance to do it. Her boyfriend was shocked by how blunt she was but it helped him understand her.