why would the detective side with main character?

There were a lot of questionable character decisions throughout film but most striking to me was the detective not pursuing charlie for any crime. firstly hes last person seen alive with his known associate co worker with no alibi and is convicted of embezzlement during same time period(motive?), then hes only semi witness to a major incident at hotel where the murder weapon of the dead co worker is found and throughout acts shifty and evasive to his questions. the detective who apparently investigates all murders in the city would no way ignore the man who's been involved in 3 separate incidents of murder . The detective sides with him in looking the other way which vsimply wouldn't happen anyway but I see no reason for his empathy with him anyway given all we believe he knows.