Pain and retribution are the least effective means of enforcing anything on humans. A man will barely cross a road to get away from suffering, but will run a marathon simply because of an idea.

If an alien intelligence that is motivated by pain avoidance comes and sees human society. They would think we are all insane and clearly have no intelligence at all. Humans invent horribly dangerous and physically punishing sports simply to play for fun. Many of us run for considerable distances just to show we can. We take ourselves back into nature to feel its' full wrath for days at a time simply to 'get away from it all'.

The instant humans started being able to think about the future concretely. We began to suffer pain for rewards not instantly apparent. Our long line of evolution has taught us that to suffer is noble and laudable. IF it leads to future gain. We even consider humans that only act to avoid pain as cowards and imbeciles.

Anytime someone claims that avoiding suffering is how religion works to check human behavior has never actually looked at how religion motivates individuals and their society. A well functioning religion holds up a vision of suffering and pain now to attain benefits later. It holds up an ideal that endures massive pain. The story always states that to live correctly you must suffer.

Religion is not offering a stick to motivate. It offers the carrot. IF it is a really smart religion. It offers that one thing biology pushed most deep into our psyche. Survival. If we suffer now to be closer to perfection. We can even beat death.

A human that feels it is suffering for no reason typically becomes nihilistic. IF to exist is to have suffering and pain(which is a truth no one can deny.) Then why not eliminate existence? This is the answer many mass shooters have even if they cannot articulate it themselves.

A good religion gives this suffering a purpose. Once the human feels there is a purpose then they gladly take the pain and discomfort. Religion simply co-opts this built in evolutionary trait of suffering for later gain to manipulate us into a functioning society.

All the religions that have ceased to be followed were based on punitive coercion. A god punished you for not doing correct things. As religions evolved they learned that didn't work very well, BUT if they promised salvation(immortality) and that suffering gets you there. Bang, they spread like wildfire. It was a meme writ large across the ages.     

I think you give them far too much credit.  Rather than seeking to give others their due suffering, most of them use the suffering as a medium and the victims as a canvas.  They are insecure attention whores who use killing as a shock 'art' to force the public to pay attention to them.  And our leftist media is happy to oblige them.