Here is a line of reasoning I have heard...
You start with this:
Murder is not in human nature -- humans are intelligent and at some core level understand that killing fellow humans is wrong. Murders happen every day. Therefore some external force is actively pushing humans to act against their nature.
(A lot of religions encourage that view.)
If you were designing something to cause humans to act against their nature it would be tangible, mysterious, and enticing. In other words it would be a real thing (a book, brick, gun...whatever) that is used infrequently enough that people develop myths about it (a car is driven every day and most people have no illusions about them...other things are actually used for their intended purposes once every few years or in a lifetime and it is easy to create myths about their uses), and actually draws people (as humans...males, mostly...are drawn to weapons).
(Again, look at mythology/religion and you will find many stories of objects that turn people to evil. It is common for antis to accuse pro-RKBA people of being Gollums, coveting their "precious" even as it turns them evil.)
You then look at what people who violate human nature tend to use or own. A quick look says that guns fit the profile and a lot of murderers use guns... so the gun is a likely cause of evil behavior. Because it is likely to be causing harm, and does no good, it should be eliminated.
Maybe evil will attach to something else...knives, motorcycles, pictures of Gore Vidal in speedos... that's part of the eternal struggle between good and evil. Banning guns was a setback for evil so evil had to regroup and choose a new vessel.