There are a few factors to consider about guns & lethal force before you "pull the trigger".
First, I'd say "have a gun". It sounds simple but you can't defend yourself or deploy lethal force w/o a loaded firearm. The recent CT school shooting incident reminds me of the Bay County Florida school board incident a few years ago.
The district's security director saw the unstable subject in the meeting but was unarmed. He had to rush to his vehicle & obtain his sidearm then run back into the room. The violent felon even stated; "he doesn't have a gun" to the sacred witnesses. The security director(a retired LE officer) shot the deranged man who later died.
Mindset is important too. I'm reading On Killing by LTC David Grossman(US Army retired). This is the 2009 version. He goes into detail about combat, death, emotions(remorse-guilt), PTSD, and other issues that are a part of killing human beings. On Killing is a highly rated book by military officials & a few federal LE agencies(FBI DEA).
As for the legal standards, I'd read or see the material of Massad Ayoob. Ayoob is a instructor, match shooter, writer, and legal use of force expert. He explains many legal & civil law issues to the general public.
I'd also know & understand your local area's gun and lethal force laws. Don't go by "bar talk" or the gun shop rangers either. See
www.gunlawguide.com or atty David Wong's legal guide to gun & knife laws. The popular site
www.handgunlaw.us helps too.
In closing, be fully ready to deploy lethal force BEFORE you carry a firearm for protection. If you are uneasy or unsure, DO NOT do it. Weapons, training AND the proper mindset are required for carrying a firearm.
Clyde