Montana Man on Trial in Killing of Teen

The homeowner says he acted in self-defense after the exchange student entered his garage at night.
3:30 | 12/05/14

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Transcript for Montana Man on Trial in Killing of Teen
talking. And now to the Montana homeowner on trial for killing a teenager who entered his garage late at night. Marcus Carmen said it was self-defense. Prosecutors say he set a trap. Reporter: In a Montana courtroom Thursday, the mother of 17-year-old German exchange student in tears. Marcus accused of killing diti in the garage after the exchange student walked in, allegedly looking for alcohol and set off motion censers and this camera. Lots of things show this case is not justifiable use of force. Reporter: He said he had been robbed before, was fearful and protecting himself when he opened fire, killing the teenager. He had to take the steps. Unfortunately to take his life. Reporter: Prosecutors painting a different picture. The defendant left the house to go outside and confront the person in his garage. We're going to hear that he trapped him in the garage, by his own words. Reporter: But he denies that, and his wife testified they were scared of the thieves who hit them. It was her idea to set up the motion censers to catch the thieves in the act. I put that out there thinking, I don't know, I could run out and be like, got you. You're staying here until the cops get here. Reporter: Earlier a hairdresser testified that he was extremely angry about kids robbing him in the days before the shooting and intent on getting them. He said he's really tired and he had been sitting up for three days waiting to shoot some f'ing kids. Reporter: He has pleaded not guilty. For "Good morning America," Neal Karlinsky, ABC news, Seattle. Okay, talk to Dan Abrams about this. What's the legal standard. If he set a trap, he committed a crime? No question. What you heard the hairdresser say is the most important thing in the context of this case. Including the potential for other witnesses. Why is that so important? Because if he was planning to do this, if he was looking to shoot kids, if he was setting a trap by putting a purse or something in the garage so that someone would come in and so that the alarm would get tripped and so he could then use his shot gun, there's no question that that's murder in the state of Montana. Called deliberate homicide. The question is, though, did that happen? Because he says that's not the circumstances. And Montana does give homeowners a lot of leeway. More than many places. Because very often, even if someone is in your home, you have to feel reasonable -- reasonably feel that you were in danger. In Montana you don't have to be protecting yourself, you can be literally trying to stop the commission of a crime. But you have to be reasonable and the force has to be necessary to stop the commission of that crime. And that's where the challenge is going to be. Even if he didn't set a trap, right, was it necessary for him to use that shotgun four times? And shoot twice, hitting him? That's going to be the tough legal question for the defendant in this case. Especially as you pointed out, the hairdresser saying he was thinking about it. If the jury believes that, it's easy. It's a closer case if they're not convinced he was planning and plotting this. And then focused on the question of was the force reasonable and necessary. Thanks very much. Go to ginger.

This transcript has been automatically generated and may not be 100% accurate.

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