In the first season of the TV show Strike (C. B. Strike) called The Cuckoo's Calling, a character called John Bristow comes to the main character Cormoran Strike asking to investigate his sister's death.
We are led to believe that John Bristow does not believe his sister committed suicide even when the police investigation has clearly concluded as such. In the end, it turns out in this whodunnit mystery that Bristow is the murderer. So what was the motivation for Bristow to ask someone to investigate the case when he clearly would have gotten away with it?
Why did John Bristow ask Cormoran Strike to investigate his sister's death?