D.S. Simpson is the officer who lead the investigations into the deaths of Stephen Mackintosh and Ali Taylor. The driver of the car that hit Ali told D.S. Simpson that Ali ran out onto the road suddenly, and he had no time to brake - a version of events which D.S. Simpson was satisfied with.
In regards to Macki's death, D.S. Simpson first decided to question Justin Burton, who was a witness alongside Darlene Taylor. Justin protested that he did not see a knife and therefore did not know how Macki ended up with a fatal stab wound. He also protested that Ali would not be the kind of person to carry a knife. D.S. Simpson was confused by Justin's version of events, especially when Justin told him that he saw a struggle. However, evidence later came to light that saw D.S. Simpson view Justin as a suspect, saying that if Ali did not stab Macki, intentionally nor accidentally, then someone must have.