Thou shalt not kill

[Gary david]

Are there any exeptions to this commandment that makes it acceptable to take anothers life? 

[chrysostom]

http://www.vatican.va/archive/ccc_css/archive/catechism/p3s2c2a5.htm

This link is to the 5th commandment in the Catechism, it's going to summarise most of what you're looking for. In short, self-defence is permitted, and when one is responsible for the safety of others, defence can be a duty. It must be proportionate to the degree of danger and so at times may involve lethal force.