@Chet31 It's probably all the guilt she felt over the years of robbing people that was already weighing down heavily on her. All of this happening probably broke her pretty hard after at least one near death scenario, her friend maybe dying because of her indecisiveness, realizing that she had effectively been put in charge of a Tyke Bomb who could mess with time and space
after thinking she signed the 'innocent' witch's death sentence, and her friend that she thought was dead came back and tried to kill Meifa- Someone she thinks is just innocent and doesn't know any better. Someone so dangerous that a Greedy Bounty Hunter who actually tried to kill Mayura, a witch that was warned to everyone to
stay away from because of how dangerous she is, refused payment on trying to harm Meifa.
Putting all that on someone within like a week or something? That would really hurt someone's mental state, especially when she was already heavily conflicted on her own morals. It was multiple feathers that broke the camel's back after Meifa doing numerous things that made her realize just what kind of danger she was dealing with. Probably thought that keeping Meifa from harming others and atoning by not letting her hurt anyone else is the best course of action.
At least, that's how I view it. A girl who was already really conflicted with her morals, getting tossed in charge of a pretty much amoral WItch who doesn't know what good or bad is due to a horrid upbringing that even a witch ended up finding gave her a bad taste in her mouth, and snapping under the pressure.