I'm not very smart, but I don't think you need to worry about drones using the linked flamethrower attachment: in order to buy from the website, the terrorist needs to click the "Agree" checkbox that says "I understand that operating a drone or UAS in the US with this attachment would require a Part 107 Waiver."
This page has some information about operating drones over people: https://www.faa.gov/uas/commercial_operators/operations_over... . That page says "the remote pilot must take steps using a safety risk-based approach to ensure that ... the small UAS is not operated in a careless or reckless manner so as to endanger the life or property of another (§ 107.23)". So IIUC, I think it's likely that the FAA would deny the application for the Part 107 Waiver, and the terrorist wouldn't be able to use it.
Hope that helps!