Byron Smith convicted himself of murdering two teenage intruders through his own words, including those he muttered into a home surveillance-type audio recorder, prosecutors argued in documents urging the state Supreme Court to uphold his conviction.

The grand jury and trial jury “did not need to hear anything else but the audio recording of the killings and his statements to the police” to determine that he premeditated the shootings, they wrote in a brief filed Wednesday.

Smith, a 66-year-old retired U.S. State Department employee, is serving a life sentence without the possibility of release after a jury took three hours last spring to convict him of first-degree premeditated murder for shooting 18-year-old Haile Kifer and 17-year-old Nick Brady, who broke into his Little Falls home on Thanksgiving Day 2012.

Smith is asking the state high court to overturn his conviction or grant him a new trial, arguing his case was riddled with mistakes — including prohibiting them from giving a complete defense, improperly closing the courtroom and failing to properly instruct the jury on a disputed claim made in closing arguments.

The case, argued at trial last April, became a flash point nationally as communities debated so-called “castle doctrine” laws and how far a homeowner can go to protect himself and his property.

Defense attorneys portrayed Smith as a man who was scared to his core after being targeted by burglars who stole guns and other property in previous break-ins.

Prosecutors portrayed Smith as a vigilante who set a trap to lure burglars, then waited in his basement and coldly executed the teen cousins as they descended his basement stairs about 10 minutes apart, continuing to shoot after they no longer posed a threat.

On an audio recording that Smith had made that day, gunshots boom, the teens are heard groaning and screaming, and Smith is heard saying “you’re dead” to Brady and “you’re dying” to Kifer. Later Smith is heard muttering how he saw them as “vermin” and that he was doing his “civic duty.”

Prosecutors reiterated that vigilante portrayal in their response to Smith’s appeal Wednesday, arguing that Smith’s actions were premeditated murder and premeditated self-defense. His appeals claims “do no more than nibble around the edges” of the case against him, they wrote. The defense’s arguments are either meritless or the points are harmless enough that they wouldn’t have changed the jury’s decision, they argued, calling Smith’s claims “an irrelevant sideshow.”

Smith’s defense attorneys argued that he didn’t get a fair trial because they weren’t allowed to present testimony from Brady’s friends linking him to earlier burglaries at Smith’s properties. The trial judge had declined to allow that, finding no evidence that Smith knew who Brady was or believed that it was Brady who had burglarized his home before.

The judge was correct in keeping that testimony out, prosecutors argued, and pointed out that Smith believed a neighbor girl was involved in the burglaries.