Perhaps the back story involves a forgotten civilization. This civilization might have had a complex and dark ritual that led to the current state of DBD. The killers could be the guardians or the punishers of this civilization's secrets.
One possibility for the DBD back story is that it is a result of a cosmic deal. Some higher - beings made a pact, and as a part of it, the DBD world was created. The killers are the enforcers of this deal, and the survivors are the ones who are being tested. For instance, the deal could have been about the balance of good and evil in the universe, and the survivors must prove their worth to survive. This would explain why they are constantly in a struggle against the killers.
Chapter 227, "There's Something Wrong with This Stepmother", is the clearest early answer because it turns Something Wrong's situation into a concrete plot event. It opens with Your Four Pillars of Destiny is very strong, with wealth, authority,...
Chapter 217, "No Problem, They're All Red Anyway", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event.
Chapter 495, "It's Not Too Much to Ask for Just a Thousand Points, Right?", is the clearest early answer because it turns Miss Su's situation into a concrete plot event.
Chapter 83, "Can Actually Carve Flowers?!", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with When they received the task, the three of them were already...
Chapter 438, "The Upper Stream is Mine, The Lower Stream is Yours", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. The chapter gives readers a real answer by opening on I'm sorry," Shang Shuyue squeezed out three words through clenched...
Chapter 297, "Is She Really His Granddaughter?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 118, "Director Sun, When Did You Get a Barrage Above Your Head?", is the clearest early answer because it turns Get's situation into a concrete plot event.
Chapter 115, "Jiang Yan's New Follower—Richard!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Jiang Yan was woken by a loud crowing noise.. By the end, [Pure bystander, wasn't Jiang Yan known as a notorious playboy?.
Chapter 281, "Zhao Lang, Are You Willing to Take a Gamble?", is the clearest early answer because it turns Chen Xing's situation into a concrete plot event.