It's primarily ignorance combined with the media/TV/Hollywood coverage & portrayal of guns.
People aren't born anti-gun or pro-gun. We learn it. Either we develop a liking for guns for whatever reason (hunting with family, target shooting, liking military/tactical things, etc.), or we don't. Most of the latter folks have no other first-hand experience with or source of information about guns other than TV, movies, and the news.
All of those media portray guns as "violent" and inherently bad in some way. Just mention guns in a normal setting, and you're bound to get some responses like, "Guns are evil!" or, "Guns are violent!" by people who've never even seen one in person. They're more than happy to assign responsibility for violence (a human action) on an inanimate object b/c that's what they see in society with guns, but not with cars or anything else. When someone dies in a hit & run, we don't blame the car. If someone tried to use that as a defense, the judge would laugh at them, as would society.
Violence is a human action, and it's been around for as long as we have. Guns didn't spark violence. People have been killing each other since the dawn of mankind, whether with bare hands, clubs, knives, swords, guns, or bombs. People disregard the fact that violence is already illegal and believe the control-hungry politicians who erroneously claim that guns are bad and create violence.
Add to it a group of people whose loved ones have suffered or died via a gun shot wound. They can't get their loved one back, and blaming the attacker doesn't take away the pain, so they add blame to the gun as though the attacker's violent nature wasn't to blame and that he wouldn't have attacked with a knife or a baseball bat if no guns existed. Then another politician uses their grief to get their vote by acting like a champion for their misguided cause of taking away our gun rights. Then the news plasters pictures of a dead child or old lady to boost their ratings, and it's a feeding frenzy of ignorance sparked by emotional reactions to violence with actions targeted at the tool rather than the attacker.
These reasons are why people are anti-gun, IMO. It's never a good thing when ignorant people get their only information from the media.