Transcript for Montana Forest Service Firefighter on Trial for Killing Alleged Burglar
You are about to meet a man who says he was similarly protecting himself when he shot and killed a teenager who walked into his garage. But this case has a real twist. Prosecutors say this wasn't self-defense. But instead, a trap, set bay trigger happy homeowner. Here's ABC's Tom llamas for our series, "Crime and punishment." Who shot him? My husband. Reporter: On the phone, Jan nel flagger, calling 911 after her common law husband shot dead this 17-year-old who entered their garage, just after midnight last April. Where was he shot? Um -- I don't know. He's laying face down, he's barely breathing. Reporter: The person bleeding to death? A german-born exchange student who walked into their garage, possibly looking for alcohol. The very next day, police arresting karma for deliberate homicide. Prosecutors say did didn't know it at the time, but he walked right into a trap. That say karma and flagger, fed up with being burglarized, set up a garage to catch the burglar in the act. Court documents show they set up a baby monitor in the garage and installed motion sensors. The question now for the jury, did karma truly feel his life and home were in danger, or was he just trigger happy? He had to take the steps, unfortunately, to take his life. Things that show that this case was not justifiably use of force. Reporter: If that's the case, why are prosecutors honing in on this bait theory, they tried to bait this burglar and shot and killed him? If they baited him, then he's guilty. You can't lure someone into your home to try to kill them. In any state with any protective stand your ground or castle doctrine law. Bottom line is, you set a trap for someone to come to your house, you're prepared there to shoot and kill that person because you've created a trap? That's murder. Reporter: Among the evidence, this surveillance image of did inside the garage. Karma says a sensor alarm alerted him so, he grabbed his shotgun, then ran outside. He says he heard threatening noises coming from inside the dark garage, so he fired four shots, striking did in the arm and then in the head. Didi's mother, leaving the courtroom in tears. Some of the images too graphic from the crime scene, including the holes in the wall from the shot gun blasts. Certainly it is easy to second guess and question what other things he could have done, but at that moment, he believed he had no other choice. Reporter: But not working in the defense's favor, many in karma's community. They claim flagger talked about how they were luring a robber. He said, yeah, we're going to beat him. Reporter: Here's the hairdresser that cult karma's hair just days before the incident. He said he was really tired and that he had been sitting up for three days waiting to shoot some Kids. Reporter: And the morning after the shooting, a neighbor says flagger told him this matter of factually. She said, you do not have to worry about the burglaries anymore and Suzanne said, why is that? And she said, because he's dead. Reporter: But karma had a history of odd behavior. His former landscaper testifying that karma came outside naked, pulling a gun on him when he set off the home's motion sensors. I was scared. I was frozen. Reporter: But according to the victim's friend, also a foreign exchange student at big school high school, they had no idea that looking for beer in people's garages could turn deadly. No one told us that you could be shot if you went inside a garage because we don't have that kind of laws in our countries. Okay, so, you don't think it was a crime to go into somebody else's house in the middle of the night? I said that it wasn't good this way and never get into a garage. I was sure that it wasn't right. Reporter: But for karma's case, one witness who could potentially do the most damage, his own common law wife. Prosecutors deciding to put her on the stand to paint the couple as a pair of vigilantes, not victims. The night of the shooting, for instance, she told police the teenager cried out, no, no, wait, please, before karma killed him. Now, she's taken the stand saying that didn't happen. I was absolute ly traumatized. I was being asked questions within a half an hour of holding somebody's brain in my hand. Reporter: While the 911 operator dreked her to give did first aid, reportedly she responded by going through his wallet and commenting, he's not even a U.S. Citizen. My heart breaks for them. I don't mean to come across that way. Reporter: Has the common law wife hurt xhar's case? Oh, yeah. If the things she had previously said happened, that's a big problem for him. Reporter: Under Montana law, a person doesn't have to feel his life is in danger at home to use force. They can actively think it would present an assault. Also known as the castle doctrine. The law has been at the center of other high profile cases, including the case of Damon verron in Texas in 2008. He, his wife and 6-month-old baby were all at home when they were burglarized. I heard a crashing through my window, which was this window right here in my bedroom and I got my gun. When I came around the corner, I saw the silhouette in my window, I pointed my weapon, I fired three times. Reporter: He told ABC news that he shot to kill in order to protect his family. He got off Scott free. If I had to do it again, I would. It's the safety of my family over us being hurt. It's a no brainer for me. Reporter: Back in Montana, karma's defense team making a similar argument. What other remedy did that have as far as -- they tahad to take the law into their own hands. They didn't have any other choice. Reporter: But all of this will be up to the jury, who will hear more testimony tomorrow. For "Nightline," Tom llamas in New York.
This transcript has been automatically generated and may not be 100% accurate.