"In general, one has to have a reasonable belief that there is going to be a deadly attack by another person," said John Junker, criminal law professor at the University of Washington. "You can't shoot somebody for running away with your stuff."
But there have been a lot of cases, some stemming from police shootings, where the shooter mistakenly thought the other person was armed. Case law says the jury must judge the facts from the shooter's point of view and the information he had, Junker said.
"It's up to the lawyers to try to prove to the jury what really happened and whether it was reasonable for him to believe that," he said.