Man it would be great if we could put the genie back in the bottle and have a world where there are no guns, knives, brass knuckles, nuclear weapons, chemical/biological weapons, and so on. But given that those things exist, and always will exist despite our best efforts to mutually disarm everyone, I want the good guys to be able to reasonably defend themselves.
In locales where gun ownership has been outlawed in the U.S., law-abiding (i.e., non-gun owning) citizens have had terrifying situations of people attempting (or succeeding) in home invasions, but the citizens were unable to protect themselves. These don't happen every day, but they do happen. They had to rely instead on calling the police and waiting for them to arrive, as the perpetrators are literally breaking down their door or coming through a window. Often the police don't arrive in time to help. I wish a well-trained, armed good guy had been in that theater in Aurora or the school in Newtown to shoot and kill those murderous cowards as soon as they pulled out a weapon.
If we could guarantee that no one with bad intentions could ever get their hands on a dangerous weapon, I'd be content to hand mine over. It's easy for me to say that since no such guarantee can (or ever will) be provided.
Similarly, I don't see the U.S. disarming itself of devastating weapons as long as countries like North Korea and Iran exist. I don't know why we'd expect the good guys to willingly disarm themselves when bad guys have weapons or an interest in acquiring them.
All that said, I do believe in "reasonable" restrictions. We don't allow regular folks to go around toting nuclear weapons or anthrax, so we all buy into the notion that there can exist SOME regulation/restrictions on owning weapons. It's just a question of what restrictions.