From the time of Billy the Kid, the right to bear arms has always been characterised as good guys vs bad guys - the implication of this being that good will always prevail.
When someone is intent on committing a terrible crime with a gun, they will source it by any means necessary - whether they are illegal is immaterial. The only way to stop these crimes is if regular, law-abiding citizens are also able to own guns and can therefore protect themselves and others from terrible crimes. Originally, when Europeans first colonized what would become the US, the population contended with a hostile landscape with only firearms for protection. Guns were the only thing that could ensure an element of safety. This evolved to not only be for practical purposes, but for recreational purposes too, as firearms became more and more popular for pastimes like target practice and recreational hunting and fishing. Since then, guns have been firmly part of American culture for fun and protection alike. Heroic American figures over time have often used guns to protect themselves and others, from Annie Oakley to hard-boiled detectives in 1920s crime novels. These figures demonstrate the powerful ways in which guns can be used for protection to stop atrocities from occurring. When it comes down to it, it is clearly not guns that are the problem. It is the people using them. If more good people had access to guns, those with bad intentions would be stopped in their tracks. If guns are outlawed, bad guys will still find a way to source them. So why should the good guys be punished and prohibited from protecting their countrymen?
The idea that a good guy with a gun can stop a bad guy with a gun is a pervasive American cultural fantasy. Allowing private citizens to carry handguns has been found to increase violent crime by 13 to 15 per cent. The world does not exist in the binary of 'good guys' and 'bad guys'. Even if ensuring everyone had guns did mean 'good guys' could stop 'bad guys' from committing crimes, this does not address all the gun deaths and injuries that are not the result of criminal intent, such as suicide and accidents. The fantasy that this would work, also, is a complete fallacy. Any number of things can go wrong with using a gun to deter a shooter, even if the intent is good. A bystander may be shot, or a criminal may just steal a 'good guy's' gun. The more people overall that own a gun, the higher the homicide rate. Those who do own a gun are more likely to be killed or injured than those who don't.
There are people who are fundamentally 'bad' and fundamentally 'good'. Individuals have the right to defend themselves by any means necessary.
[P1] Bad guys have access to guns and are going to commit crimes with them. [P2] The only realistic way to stop this is to ensure good guys also have guns in order to protect themselves.
Rejecting the premises
[Rejecting P1] If 'bad guys' have guns, the government should ensure they take access away from them. [Rejecting P2] Giving 'good guys' guns does not necessarily ensure safety.
Henigan, Dennis A. "Guns Dont Kill People, People Kill People": and Other Myths about Guns and Gun Control. Beacon Press, 2016.