According brother Eli's teaching, a law officer, Police or Army, has the duty to apprenhend a criminal and bring him before the law. Not to kill.
According to Luke 3:14 (KJV)
14 And the soldiers likewise demanded of him, saying, And what shall we do? And he said unto them, Do violence to no man, neither accuse any falsely; and be content with your wages.
Get your training well and learn how to apprehend a criminal without killing him. He could be disabled in order to catch him.
However, this is not in negligence to self defence.
God is the owner of life. He give life and He takes life.