I took a CCW course a couple of weeks ago in which this new Missouri law was explained. Basically, it says you don't have to retreat if you are in your home, car, motel room, tent, etc. BUT you still have to ascertain that the intruder has opportunity, means and intent to do you, or someone else, harm, i.e. it has to be a legitimate threat. If someone is stealing your stuff and is on the way out you can't shoot. We may wish we could just shoot a burglar, but the fact is that we can't shoot just to protect property; there has to be a legitimate threat of bodily harm.
The law DOES NOT shield you from civil suits - anyone can sue you anytime for anything. Whether or not they win, of course, is another matter. What the Missouri law does is buy you a jury instruction to the effect that if they find the shooting was warranted under the law then they may NOT find you liable for civil damages. Moreover, the law says that if you win the civil suit, then the plaintiff has to pay your costs and court costs, so it should reduce the number of such suits, but it does NOT prevent the filing of one.
Hope this helps. Understand that I'm not a lawyer, but this is how it was put to us in the class.
RonF