A jury on Tuesday found Byron Smith guilty of the murders of two teenage cousins who broke into his Little Falls, Minnesota, home in 2012.
Smith, 65, was found guilty on two counts each of first and second degree murder in the deaths of Nicholas Brady and Haile Kifer.
He was immediately sentenced to life without parole. His attorney said he would appeal.
"It's not a fair or a just verdict," Steven Meshbesher told reporters. "He is adamant about an appeal and he has every right to that appeal because there are a lot of things the jury did not see."
Smith has admitted shooting and wounding, then killing the intruders. His attorney had argued that Smith acted in self defense and out of fear for his life when he killed 17-year-old Brady and 18-year-old Kifer.
While the two dead teens were not armed, Smith has said he was afraid he'd be killed with a shotgun that was stolen in one of several previous burglaries.
It's legal in Minnesota to shoot and kill someone in your home if you feel you're in danger, or to stop a felony in progress. The law says there's no duty to retreat.
Prosecutors say Smith crossed the line between self defense and murder. They say once Smith shot and wounded the teens, the threat was over and he should have called police. Instead, he kept the bodies in his basement before calling a neighbor the next day.
Jurors had to balance the testimony they heard about Smith's upstanding character against all that they saw and heard last week: autopsy photos of Brady and Kifer after they'd been shot at close range; audio Smith recorded himself of the break in and shootings in which he was heard saying the teens were vermin; and Smith's interview with investigators in which he said he killed Kifer with what he described as a "finishing shot."
The court made the audio recordings available to the public.
The jury took about three hours to reach its decision.
Kimberly Brady spoke to reporters after the sentencing. "My son was a tremendous kid," she said. "I think often of what he could have been and I see other young men with their dads or their moms and it's really, really difficult, and I have to think that I never will have that chance again."
Court documents that were not allowed as evidence showed Brady had broken into Smith's house and garage before. Brady and Kifer were also linked to another burglary the day before they were killed; stolen prescription drugs were found in the car they were driving.
Judge Douglas Anderson excluded evidence about the teens' histories from the trial as irrelevant.
The Associated Press contributed to this report