Whitman didn't understand why he killed his wife and mother, then went on to the college. He stated as much in a letter and asked for an autopsy to see if there were physical damage to his brain which would cause the headaches he was having and the irrational, violent thoughts. It was found that he had a brain tumor.
"love does no harm to a neighbor" (in Romans 13:10). I don't think someone can hurt someone else, unless the person's character is capable of hurting people. But there are secular people in psychiatry who do not believe in the existence of spiritual beings who can cause people to do evil. And they do not buy that we humans have a spiritual level of being, deeper than our physical brains, from where can come our drives through our brains to do what we do. If you love someone, pain can not get you to hurt or kill someone you love.
And, "of course", was Whitman telling the truth? And was he being deceived by an evil spirit . . . of Satan's deception? Jesus says people will kill us Christians and think they are serving God. So, yes humans are capable of believing what is not true so they can then kill and justify it.
When I was a kid, we had our toy guns, but in our games we only killed "Indians" and maybe "robbers". We understood we were never to shoot anyone who was innocent and not trying to hurt us. Now, of course, I see that the world's representation of "Indians" had nothing to do with how our native tribal peoples have been, and who was the real aggressor . . . and enemy, perhaps I could say.
So, I would say that psychopaths are capable of killing ones who are no threat to them. I understand there are killers who do it for what they find to be entertainment; they don't need guns.
Also - - psychopaths, in history, have often been clever enough to use whatsoever has been available in the culture where they have been . . . including false accusation so ones even in judicial religious power would do their dirty work.
And there are psychopaths who kill because they can't get their own way > this can include killing their own selves if they find they have no hope of having the lives they want in this world.
And God does resist the proud >
"God resists the proud," we have in James 4:6 and also in 1 Peter 5:5.
When God's resistance effectively shuts down a selfish person's life, then is when the person can become "desperate" and kill others and/or kill oneself. So, when I start to get bent out of shape about things not going the way I want, this is all I need to know > I need to stop and wait for God to correct me and restore me into sweet and sensitive sharing with Him in His peace, then discover what He has me doing in goodness of His peace, and enjoy.
So, the problem isn't the guns or bad brain chemistry.