Sean will doubtless correct his view when he thinks about it. The DSS is another matter. I felt right away that the prospect of sticking the stepfather with a murder rap was very much a factor in DSS thinking. At the same time, it would have buried the story and covered up their past failures to protect the child despite sixteen reports of her being abused. They're figuring if Haleigh dies, that whole ugly story of earlier abuse gets overshadowed and forgotten, and the DSS gets off scot free.
I agree with your theory on the DSS's motives.