Legal Question in Criminal Law in California

California does not recognize the right of an individual to use deadly force to protect personal property, and use of force to eject a trespasser is extremely limited. (Of course, California had 1,080,747 property crimes in 2008 alone.)

Use of a shotgun would constitute deadly force. In order to defend yourself, you must have an honest and reasonable belief that you are in imminent danger of death or great bodily injury from an unlawful attack, and that your acts are necessary to prevent the injury.

When defending ourselves or others, the law allows as much force as reasonably seems necessary under the circumstances. That will sometimes include deadly force. (Deadly force is never allowed when only property is at risk.) There is no rule that always forbids shooting an unarmed attacker. It usually wouldn't be reasonable to do so, but sometimes it would.

Whether it would have been reasonable here depends on the circumstances. If it seemed reasonably likely that the attacker would kill or seriously injure someone (you use the word assault, but not all assaults are equally serious), then your neighbor could have responded with deadly force if he had no better options readily available.