I came across this a few years ago, and thought it was great. When a friend posted a similar fanfic (the one about the old lady who thinks a demon is her grandson), I went hunting for it again. This version is slightly different, but it has and entirely new (to me) part II (the actual wreaking of the mayhem).
https://www.reddit.com/r/tumblr/comments/3k5tmm/insurance_company_fanfiction/
The Virtual Jackdaw 