The Fifth Commandment is one of the most complex and troublesome sidequests in Kingdom Come: Deliverance 2. However, it's a necessary step to reach one of the best possible endings in the game, as well as to romance Katherine. If you're having trouble solving the murder of Katherine's friends, here's a detailed guide on this troublesome quest, including all the information about the investigation, which dialogue choices to pick, and how to catch the Serial Killer.