It was the relief postman who was killing people. He 'had a fling' with the policeman he shot and tried to get Wallender to notice him. The policeman was 'in love' with Wallender.
By reading Wallender's daughter's postcard he knew she would be at his house and when Wallender eventually got there she was being held with a gun to her head.
Wallender pleaded to be shot instead (After admitting being a crap dad) the young subordinate copper shot the postie ,went out and threw up. Final scenes at the policeman's funeral, Wallender placing blue flowers on the coffin. I'm hooked! will read books now.