I find it fascinating that some people have never themselves or known anyone else who, upon some person causing them serious loss of emotional well-being/time/work/money, joked about harming them in some way while venting to a friend or relative without any intention of doing so.

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.