Why did David Butler quit his job?

David, a former policeman, resigned from the force after a judgement call resulted in him shooting and paralyzing a white teenage. When he instructed the young man to stop, the young man reached into his backpack to grab something. As a result, David, who feared for his own safety, decided to pull the trigger. In the end, we learn the boy was actually reaching for his inhaler. David resigned soon after.