I find it rather surprising, but generally it is a position based almost entirely on fear, and not on fact. They may well be people who are generally rational in their life, but when it comes to this issue fear and propaganda motivate their position, not facts and logic. They want guns banned because they are scared of them, not because they've done any research and concluded it would make things safer.
You can clearly see it in the grandparent post. Not only the name calling, but the complete detachment from the reality of things. The fact that he believes that a small group of crazies are synonymous with the greater gun owning population. Same deal with how people will generalize the nut jobs at the Cliven Bundy ranch to be the greater gun owning populace.
None stop to think that around 40-50% of all households own a gun in the US, meaning that you know someone who owns a gun, even if you don't know it, and that if that behaviour and thought were the norm for gun owners it would be rampant rather than aberrant.
They are the same as people who will point the finger at religious or environmental extremists and declare that all people of that religion or viewpoint must be extremists and scary.
It is sad, because an informed debate on gun control could be very useful, but it is really hard to have when so much of the "control" side is actually wanting a ban and the reason they want it is fear, not logic. They don't do any research, except maybe to try and look up numbers that support their view. They don't want information, since emotion is the driving factor.
Hence, name calling, scare rhetoric, and so on.