There is nature and nurture.
Our outcome and how we turn out depends on both...sometimes more one then the other.
Nature: You have three kids all grow up with the same parents and in the same house in the same neighborhood. Only one turns out to be a murderer. For that kid, his head was genetically wired to be a certain way. Making some assumptions, his/her environment and parenting wasn't so much an influence on the outcome.
On the nurture side of things, you can take someone who isn't so fucked up in the head, and expose him/her to a lot of bad shit. Abuse in the home, poverty, ..etc. And this effects how he/she CAN turn out. But it isn't a given nor a guarantee that they become a murderer.
IMO you take someone who has a genetic predisposition to be a certain way and combine it with just enough bad nurturing, and you have a fucking problem.
Still, these bad apples should not be the reason why our Bill of Rights are taken away as stated in the 2nd Amendment. Which wasn't to defend against a thief breaking into your home. But to enable our ability to form militia against an oppressive government should it ever occur.
Maybe a few hundred years ago, I had a rifle and some guy in the military had a rifle. Now, you have your arsenal at home but it wouldn't compare to what the military could bring down on you today. Perhaps if every citizen with a shotgun turned on the government all at once maybe they could do it. IDK.