In the interest of full disclosure I do want to make mention that what follows has been edited slightly, simply to make it more understandable. I don't take a great deal of time to edit my FB comments when I post them.
While on FB one day, I came across a question posted by a man about his son. It seems his son had turned to atheism because he simply couldn't understand how a loving God could permit such atrocities as mass murders. This man was asking Christian people what to say to his son. At the time I wrote this, no other Believer had offered so much as an "I'm sorry for what you're going through." I was the only person who offered any help with his question. Agree with my answer or not, I understood this father's heartbreak and did what I could to help.
Hi, _________, In short, God doesn't stop people from killing others for one simple reason: free will. Just as God gives your son free will to reject Him, God gives free will to everyone to spend as they choose. Your son would be furious if he were forced to love and serve God against his will, wouldn't he? The only 'fair' way God can treat people is to allow them to live as they choose, rejecting Him, killing others, and the like, or serving others, and accepting Jesus as Savior. Those who choose Jesus, choose life. Those who reject Jesus, choose death.
It's irrelevant that the consequences of your son's choice to reject God don't seem as 'bad' as the consequences of killing others. This is the limited human understanding of morality and the sum total of consequences. Humans see the murder of others as worse than atheism, but to God a lost soul is a lost soul. On the surface, the consequences might appear different but here's the truth: Your son, having chosen to reject God, has NO IDEA how many people will never find a saving faith in Jesus because of his unwillingness to share his faith In Jesus with them, and/or his willingness to spew his hatred of God onto others. Without having accepted Jesus as Savior, these people will spend eternity in hell. While your son did not end their earthly life, he certainly helped to prevent their eternal life in heaven. What if God considers this behavior murder? (Matthew 18:6. I know Jesus is speaking of children in this scripture, but aren't we all God's children? And I know for a fact that atheists work very diligently to pull people away from God.)
One final thing: Atheism is based on subjective truth, what each person feels is right for themselves. Subjective truth is fine for them, and you, but only if you agree with them. It is life outside of a theistic worldview because objective truth and theism go hand in hand. At best, atheists are hypocrites because while they want one thing for themselves, they deny the same freedom to those who disagree, all while borrowing the moral construct presented in the Bible. So, while your son thinks he's dodged the hypocrisy of Christianity (how can God allow/commit murder when He can stop it), he has only swapped his life of faith, his very salvation, for the true hypocrisy of atheism. This sad truth is devastatingly illogical.
It's also a problem for two reasons. First, the hypocrisy of atheism allows a person to see themselves as morally superior to everyone, even God, but, by their own definition, morality is personally determined, which means nothing short of anarchy can reign.
Second, atheists overlook, because truth is subjective to them, that God has in place a judgment day for each and every person. Those who kill others will NOT go unpunished, atheists included. Your son is conveniently overlooking the fact that those who do such horrible things will pay for them.
No amount of subjective truth wielding changes objective truth. That it can't be changed is the very reason it's objective. Atheism is simply a lie in which people wrap themselves so that they aren't offended by God. Atheism is the SELFISH, IMMATURE lightweight response to truth. The MATURE life-altering response to the objective truth presented by the Gospel is to admit that you can't save yourself, much less anyone else, that morality does not begin and end in you, and that you need to face the ugliness within your own heart so God can cleanse you and make you like Jesus. When this happens, objective truth sets you free from the prison of sin, from hypocrisy and from eternal life in hell. Win, win, win.
I hope you find my words helpful. My answer is my earnest attempt to take your question seriously.
Thanks, Gerald Altmann for the pic!