If gun sales are restricted further the people that intend to break laws will probably still get the guns. A killer certainly isn't going to have any moral dispute or fear of the law when trying to obtain a firearm considering the crime they wish to commit is much worse than getting the firearm illegally.
Now the moral good citizens are having problems getting firearms to protect themselves and their families. Since law enforcement can't be everywhere at once or in time to save lives people have the right to defend themselves.
PS (I have no affiliation with any particular political party

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