Basically I think she is terrified that she might not know where the line goes; that she can't tell which paths might lead to that darkness, so she shuts all of them off. And if she doesn't know what causes it, or how to separate out that part of her, no wonder she comes up with blunt solutions (NO guns!)

This will be a fascinating character arc (if this is indeed where Chibs is going) but I can't help but think it's rather unfortunate that they chose to do this at the same time that they cast the first female Doctor. :/