In your home, I think it's a no brainer. If you're in my house or on my property, and you are there illegally, or in the process of committing a crime in my house or on my property, then I consider you fair game and I should not be held criminally or civily liable for anything that occurs afterward. Your home is the one place that you should always be safe. However, the flip side is how many in the home shootings will take place because of an "argument" or some other dispute between family members or friends who were at one point, invited into the house, things went south, now the resident claims "Castle Doctrine"? Could get weird, but if it's a legitimate attack on your home, you should be able to do whatever you need to do.
In public, I think the "rules of engagement" should be pretty clearly defined, which for the most part, there is an acceptable level of response that we all deem reasonable and necessary. I do not think that someone should be required to retreat if they are lawfully someplace that they are allowed to be. Especially if being physcally confronted or threatened.
My concern is that some folks who may be a little less responsible, while in public, can turn a minor situation into something deadly and have it called justified. I think that if the investigation concludes that the use of deadly force was justified, then that person should be absolved from any criminal or civil liability. I can just envision a near miss in a parking lot somewhere, somebody responds with "hey, what the F is your problem buddy, you almost hit my kid" and next thing you know, rounds are being fired because the guy in the car felt "threatened" over what would have otherwise been a reactive verbal exchange and nothing more. In public, I think it opens up the possibility up for 1000's of scenarios that can go horribly wrong.
A good friend of mine was involved in an on-duty shooting not to long ago. The suspect died. He was cleared by the prosecutor and recently, his department. Now they face the possibility of a civil lawsuit. Anyone who takes the life of another, whether justified or not, potentially faces a civil lawsuit. And I don't think that's right, especially if it's justified.
So my take; Home: You're fair game if you're a bad guy, no questions asked. In Public: If you're cleared and it's deemed justified, you go on your merry way.