By Lori Grannis

MISSOULA, Mont. (Reuters) - A jury began deliberating on Tuesday over whether a Montana homeowner committed a crime when he fatally shot a German exchange student who entered his garage, in a case testing the state's version of a "castle doctrine" self-defense law.

Markus Kaarma, who prosecutors have painted as an armed aggressor who lured a would-be burglar to his death, is charged with deliberate homicide in the April death of Diren Dede, 17, of Hamburg. Authorities say Dede was killed while "garage hopping" at night in Missoula in a possible search for alcohol.

In closing arguments, prosecutors described the German high school student as an unarmed child who was “violently and senselessly executed” by a man seeking revenge for the earlier theft of objects, including a favorite marijuana pipe.

“You cannot claim justifiable homicide if you’re the aggressor,” prosecutor Karla Painter told a Missoula jury.

Prosecutors argued during the trial that Kaarma had installed motion detectors and a baby monitor days before the shooting, and left a purse filled with cash and other items in the garage on the day Dede was killed.

Painter argued that a homeowner “must actually believe danger exists” to claim self-defense of an occupied structure, and she contended that when Kaarma left his house to corner Dede in the garage he lost legal protection under Montana law.

Prosecutors also said ballistic evidence showed Kaarma had repositioned himself for a final kill shot of an already wounded, weaponless Dede, whose killing sparked outrage in Germany.

“He took three shots, then adjusted his aim. That’s the sickening part,” prosecutor Andrew Paul said. “He adjusted upward to shoot him in the head.”

Defense lawyers counter that Kaarma acted to protect his family and that his actions were in line with Montana's self-defense law, which allows deadly force against a home invasion if a person reasonably believes it is necessary to prevent an assault.

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Defense attorney Paul Ryan said the Montana law, dubbed the "castle doctrine," did not require Kaarma, 30, to hide or retreat from an intruder in his attached garage, which Ryan said was considered an "occupied structure."

"We don’t have to prove anything. It's the state's burden to prove every single element," he said. "That means you have to dispel every single doubt that comes up in this case."

Kaarma, a former U.S. Forest Service firefighter, faces up to 100 years in prison if convicted.

(Reporting by Lori Grannis; Editing by Cynthia Johnston and Eric Beech)