The standard has been based on what the officer or citizen knew at the time, provided the officer or citizen met a general standard of being a reasonably competent person. Those who seek to change the law have done so under the misunderstanding that officers do not currently have to use lesser means of force when such means are appropriate. In other words, police currently can't legally shoot a fleeing shoplifter, but they can chase and forcibly take the shoplifter into custody, using as much force as is required to overcome the suspect's resistance. If an officer knowingly and improperly uses deadly force, he is criminally liable and there are ex-cops in prison who prove the point. So, it appears on the surface that there is no real change in the law. The difference is subtle but substantial. Though the application of the proposed California law is still unknown, it appears it will include a change in legal perspective. Instead of weighing the officer's knowledge at the time of the shooting, the law will probably allow for a judgement of whether the officer's belief was accurate.
Let's say that an officer is chasing a man down a dark alley, the man reaches toward his belt pulls an object from his belt, and turns pointing the object toward the officer (this happens with staggering frequency). In that split second, the officer must decide if the object is a gun. If the officer waits for more information, he is depending on the suspect missing with at least his first two shots. What if it is a cell phone, which as I said, happens frequently? I suppose that is a risk the suspect must be willing to take. If you wonder why anyone would try to fake a gun against an armed cop, ask elsewhere because I don't know, but it has been caught enough on video tape to demonstrate that a population of idiots expects police officers to run away when an object is thrust at them. As I said, if you are willing to run from the police and ignore their lawful commands for you to stop, and if you think that it is prudent to pretend you have a gun, you must take your medicine.
I suppose less than 0.5% of the population can win a quick draw contest if the other guy draws first. Try it. Point your finger at a friend and have him keep his finger down to his side. On his own initiative let him quickly raise his finger to your mid-section and say "bang." You try to say "bang" as soon as you see him draw. You can't shoot early, that's murder, right? After you've done that a few times you will discover that by the time your eye tells your brain that danger is approaching, and your brain tells your hand to fire, it is already too late.
Any young person who is considering a career in law enforcement should look somewhere other than California. If you're currently working, brother, stay in the car.
In Practics: Handgun Defense System we talk a lot about the use of force and having an honest regard and practical grasp of law. If you have a gun at home for defense, you need to know the law that governs where you live. It's NOT just common sense. Speaking of the book (which is my job), please remember that it is not a first time shooter's book and is not recommended for new gun owners. There are much better choices if you're just beginning with handguns. Of course, I would like you to read The Perfect Pistol Shot but that marksmanship book is not really for brand new shooters either. Foundational operations and safety training are mandatory for every gun owner.
Home defense is your national birthright but like every right, it has a corresponding responsibility. Know your legal obligations.