answer: My answer would be that people around the world remember world war II with V1 and V2 missiles raining down on London during the war there. They also remember the Firebombing of Dresden Germany, Tokyo and many cities in Japan to make the Japanese Surrender and then finally the A bombs of Hiroshima and Nagasaki.
They associate Drones with all these things and drones are scarier because no human being might be involved in any of this at all. This is why this is such a scary thing about drones.
Two movies that might help you understand better this are:
EYE IN THE SKY stars Helen Mirren as Colonel Katherine Powell, a UK-based military officer in command of a top secret drone operation to capture terrorists in Kenya.
... Also starring Alan Rickman, Barkhad Abdi, Jeremy Northam, Iain Glen and Phoebe Fox, the film is directed by Gavin ...
Eye in the Sky is a 2015 British thriller film starring Helen Mirren, Aaron Paul, Alan Rickman, and Barkhad Abdi. Directed by Gavin Hood and based on a ...
And the second movie is:
Oblivion is a 2013 American post-apocalyptic science fiction action film based on Joseph Kosinski's unpublished graphic novel of the same name. The film was ...
Both of these movies lead to the unease of mankind around militarized Drones.