Despite his friendship and respect for Crixus, Doctore didn't spare him from the full pain of his lashes. Not only did his duty outweigh his friendships, but as a true teacher, he had to enforce consequence to a man who was still his student.
In ancient Roman Law, any guard or Centurion who loses or misplaces what they are assigned to watch was put to death; this was why Hector was killed after it had been discovered that he lost the gate key.
Crixus endured 17 lashes when he was punished.