Neighbors shot and killed a man violating a restraining order
Unless I woke up in the wrong country the last I heard neighbors are not in the business of enforcing restraining orders. They will need to prove they had a reason to intervene, such as he was beating on the woman at the time.
Does yelling "Come on out. I'm going to kill you" count? Somebody says they are going to kill someone, and there seems to some immediacy in the declaration, I'd say that's sufficient.
However as others have pointed out, he, his wife and the woman the guy was after, should have bunkered up and waited for the police, if the police got there quickly enough, fine, if the guy came on again, then they could both shoot him.
But they had guns, in California, the coastal part, they're toast.