PHOENIX — The video shows what appears to be an incredibly heinous act of premeditated murder

Nearly a dozen innocent bystanders scream and yell from the other side of a dry wash, but it's not enough to keep a man from stabbing a woman to death. Investigators say the man is 25-year-old Fernando Acosta, and he is being charged with first-degree murder and aggravated assault. 

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Witnesses said it could have been much worse if one bystander didn't grab a gun from his car.

"Shooting towards the ground to see if he would stop, but he didn't," said another witness, Gustavo Munoz. 

Eventually, Munoz said the bystander with a gun fired at the suspect, dropping him to the ground and ending the threat to anybody else. 

It's an act most reasonable humans would be in favor of: firing a gun at someone to try and prevent murder. 

This bystander isn't being charged, and police aren't even releasing his name, but the rare situation does raise a question about legality. When is it acceptable to shoot someone in Arizona if they aren't posing a direct threat to yourself?

"Make sure you are doing the right thing, because if people are just screwing around or it's just a fist fight, you don't know the right to bring that deadly force into the situation," said Valley attorney Monica Lindstrom. 

The key is being able to convince a judge or jury that any reasonable person would do the same thing. 

"If you need to use deadly force to protect yourself, you can also use deadly force to protect a third person as long as a reasonable person would do the same thing and you believe it's immediately necessary," Lindstrom says.

There is a catch though, and it's why you need to be able to live with whatever consequences come from pulling the trigger. 

Just because Arizona law protects you in these situations, doesn't mean the person you shoot can't sue you. 

It's much easier to prove someone civilly liable than legally responsible. Civil cases won't land you in jail, but they can be financially crippling to you and your family.