On the other side, she did write about committing a school shooting. Its hard to infer comedy from a text chat you have no context on. If you pretend this info did not come from corporate snooping then I have nothing against the FBI investigating this.
When someone has seriously harmed you in some fashion, it's pretty normal to fantasize about harming them back in some way (without intent to actually do so) as a coping mechanism.
This is a teacher in the US who would have had to sit through live shooting drills and never believed a live shooting would happen at her school (How much would you need to be paid to work somewhere where you seriously believed you had a 1 in 100 chance of being shot in the next 5 years, it wouldn't be a teachers salary for most of us).