If you are a security guard , police officer or army in the force of any nation your duty is to protect lives and properties, that's the first and foremost duty of a security guard or those in the arm forces, like police, army, navy .etc.
It's your duty to protect other people's lifes and yourself from criminal and that's why you are given a gun to fight against criminals coming to you with guns too.
In th Bible self defense isn't a sin, it's very well recorded in the book of Moses, that if you are able to kill a man because of self defense, you aren't liable, if any man accidentally kill somebody in defense of his own life, God prepared a place called a place of refuge.
Number 35 Vs 10 To 16.
Speak to the children of Israel, and say to them: ‘When you cross the Jordan into the land of Canaan,then you shall appoint cities to be cities of refuge for you, that the manslayer who kills any person accidentally may flee there, they shall be cities of refuge for you from the avenger, that the manslayer may not die until he stands before the congregation in judgment....Numbers 35:10-12
They are people who may kill another unaware or accidentally, that's why God appointed 6cities in Israel, those who kill accidentally will run there to seek refuge to avoid vengeance from those who know the person killed.
God explicitly teaches us that if you have killed any person unaware, while defending yourself, God provides a place a refuge where you wait for confrontation to prove that you are innocent.
Anyone that kills in the city of refuge from vengeance must be killed by stoning.
This means that God justice dictates that if you kill anybody protecting lives , it's not a grave sin in God's sight.
Prophet Eli teaches that the police, army should be sharpshooters , when faced with danger do not shoot the person in the head or chest, but instead disable the person, so that he cannot run away.
It's better to be compassionate even to criminals, that is the orientation in God's word.
Also , try as much as you can to catch a criminal without killing him, let the law decide his fate.
