"The last thing I want to do is hurt him. But when death is the alternative I've got to at least try." She didn't think she could handle if she didn't try and Stiles ended up dead. The what-ifs would be torture. "I'm not going into this like its a game or anything. I have a certain task and that's what I'll be focusing on."
Maybe it would be futile in the end, but she couldn't not try. It wasn't in her nature not to and she doubted Stiles or Lydia wanted one of the Originals to do it. Otherwise they would have asked one of them, but they had come to her.