Since logging into the backdoor account produced a `#` prompt instead of `$`, it was uid 0, so the last will and testament was either in `/root` or in `/`, depending on how the non-backdoor root account was set up.
Plus, if Flynn was running those commands while logged in as "backdoor" rather than while logged in as "root", the text displayed on-screen specifically says that the backdoor account doesn't have a home directory configured so it would treat `/` as the home directory. Which would mean the computer now has a `/last_will_and_testament.txt` file. That's pretty prominent and attention-drawing. It's going to be found by anyone who investigates that computer.
Plus, if Flynn was running those commands while logged in as "backdoor" rather than while logged in as "root", the text displayed on-screen specifically says that the backdoor account doesn't have a home directory configured so it would treat `/` as the home directory. Which would mean the computer now has a `/last_will_and_testament.txt` file. That's pretty prominent and attention-drawing. It's going to be found by anyone who investigates that computer.