Conflict of Duties

The axe murderer was an example used to describe a conflict of duties with the sense that it is a person's duty to be honest but also keep the friend alive.  I could very well see an argument being made for school shootings however I have problems in understanding the duty.  Hypothetically speaking, if a teacher were to have been armed on that campus would not his only duty have been to keep safe his students to the best of his abilities. From class I understand that there is a duty to not take the life of  the gunman but also a duty to save the lives of the students.  If in defense of his students, the teacher had to kill the gunman before harm was done then he would have been justified in doing so in a court of law here in the United States.

Duty 'is the necessity to in act in reverence of the law' yet if this was the case then the teacher should not shoot the gunman, as it is murder and against the law.  Rather sit back and try his best to protect the students from a man with a gun who is intending to murder them.  I cannot agree with this philosophy of duty and neither do a lot of Americans who believe in self-defense and the 'stand your ground' laws.  If someone truly possesses the desire to rape or kill an individual or group of individuals and is about to execute that desire than it should be the duty of others (assuming they are in the proper position to do so) to kill that person and prevent from doing harm to the other.  That is the only duty in a kill-or-be-killed scenario that was the Oregon shooting.