Frankly...this is wrong, in my opinion. First off, the accurate translation of the "killing commandment" in the Bible is "You shall not commit murder" (as opposed to kill), and there are NUMEROUS passages in the Bible describing violence for a righteous cause or in self defense. Heck, Jesus even told his own disciples to carry swords!
Second, since you're in confession, I'm assuming you're either Catholic or Orthodox. I know in the Orthodox church we believe ALL sins can be forgiven. I'm assuming Catholics are the same way.
Read:
http://www.christianity.com/faith/1348211.aspx
You don't need to feel bad about anything you've done or may have to do.
well the bible can be interpreted in different ways.. but your basically saying the same thing that the priest said.. its not murder its self defense... and yes im catholic.. i dont feel bad about what i did.. i did it because people where shooting at me and i returned fire.. i'll do it again if placed in the same position.. where my life or the life of my loved ones are being threatened.. and i wont regret it..