Lets say future-murderer Bob wants a gun to shoot his neighbor with.  Lets say, all gun sales and ownership is now illegal.  So, Bob goes to his good friend he met over the deepnet, Jane, who is selling guns under the table at a markup for the risk involved.  Bob acquires his gun through breaking the law, and goes to his neighbor's house.  His neighbor, George, is not armed, because guns are illegal, and George is a good law-abiding citizen.  Bob breaks into George's house, and meets the unarmed George.  Because George is unarmed, and Bob can kill him at any moment with his own gun, Bob then rapes George, and after, shoots him dead.

Later, the police arrive (who are allowed to have guns, btw), called by the neighbors only after they heard gunshots, and arrest Bob, who was sobbing in the corner.  Thankfully, due to the law against guns, Bob got extra time in prison for gun ownership, and George...well, George is dead, so that really doesn't matter to him.  Thankfully, however, the law against gun ownership was there, and this crime did not go unpunished--I mean, Bob is already getting life for raping and murdering a person, and he would've been dead even after the gun law charges were put into place...