The guy made a deal with a devil where he must kill 666 people in 666 days or else his daughter will die (she is in the hospital after getting hit by a truck).
But the guy is conflicted so this is his compromise.
For me, I (and maybe other readers) knew the "rule" of this series after 2 chapters, so whenever there is a new episode I'll just read the first page, think, guess the answer and read the second page afterward to see if I was right or not.
I treat this series as a guessing game and for me, it's fun to read it that way.