Dede, who was unarmed, was shot dead with a pump-action shotgun on April 27th this year.
The killer, 30-year old Markus Kaarma, had claimed he was acting in self-defence under the state of Montana's "stand your ground" law, after finding Dede in his garage during the night.
However, jurors believed the prosecution case that Kaarma had set a trap after having his garage burgled twice before.
There were emotional scenes in the courtroom in Missoula, with Dede's mother breaking into tears as the verdict was announced.
His two sisters had watched the trial live online, according to Hamburger Abendblatt newspaper. They told the paper the verdict was "a relief - but not a joy."
Dede's father said "Everyone needs to get the punishment they deserve."
The prosecutor admitted Dede - who had no reason to be in the garage - "was a teenager who made the mistakes teenagers make", but said Kaarma had executed him in cold blood, in revenge for the two previous burglaries.
The court heard Kaarma left his garage door open, with a purse visible, as part of the trap. Police testimony showed that Kaarma may have even paused to adjust his aim before the third and fourth shots.
Kaarma was led away from court in handcuffs, and will face sentencing on February 11. He faces a minimum jail sentence of 10 years, with a maximum of up to 100 years.
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