After crime rate falls to zero, a lonely detective turns into a serial killer to challenge his fellow detectives.

If I understand correctly, the major question here is the following: Given the audience want to identify with the main character to come along on the ride, what did you imagine will make this person accessible. It's not even about being a serial killer, it's more of a general requirement. With other characters it's just easier to see or implied.

ashes123:

Whether the detective has a motive? was not my point.? ?My point was to wonder whether there is a movie audience for story about a man who literally goes from "good cop"? to "bad cop", who intentionally kills innocent people, violating not only the law but every socially acceptable norm. And for no redeeming purpose.

ashes123:

Whether the detective has a motive? was not my point.? ?My point was to wonder whether there is a movie audience for story about a man who literally goes from "good cop"? to "bad cop", who intentionally kills innocent people, violating not only the law but every socially acceptable norm. And for no redeeming purpose.