I would really feel more secure having one and just need to know when it's okay to shoot it without getting assault/murder charges

I hope you understand that it would be a false sense of security.

If you had had a gun in your car when you came home and encountered the thieves, and had been so foolish as to approach them with it, the chances are very high that you would be dead now.

You think the thieves didn't have guns of their own, and far more motivation to use them than you had to use yours? Not to mention far more skill and experience in their use?

One of the luckiest things in your whole life is that you didn't have a gun,

tanstaafl.