The story begins at this page Atonement .
Kabir Singh was promoted to detective due to his excellent capability to think like a criminal. He had a masters degree in Criminal Psychology when he joined the police force. His father, who was also a decorated police officer, was gunned down at a robbery gone wrong which is why he joined the force to make sure that no one else met that fate. He hated all criminals no matter what the gravity or the reason of the crime was but ironically he had the ability to connect with the criminal mind. This family slaughter was not some meaningless killing. It was a hate crime towards the nature of the business of the deceased.