Hell's Half Acre

She did it to buy time. She needed to get out of there before John saw her - if she had let Sherlock engage her in conversation, John would have heard the voices and/or made his way to the room before they were finished. She had to get out of there before John saw her. I think she knew there was a possibility that Sherlock might die, and then her life would be easier because her secret would die with him - but she didn't actually WANT him to die, because he's a friend, so she did everything she could to stack the odds in his favour, even though she probably could have lived with either scenario - she IS a fellow sociopath, after all.