(CNN) Reat Griffin Underwood was heading to a high school singing competition when a gunman shot him dead.

On Tuesday, his voice rang out again. This time, it echoed in a crowded courtroom as a judge weighed whether the man who gunned down the 14-year-old, his grandfather and another victim at two Kansas Jewish centers last year should pay for the killings with his life.

As relatives of the shooting victims made their case for why Frazier Glenn Cross should receive the harshest sentence the law allows, Underwood's mother, Mindy Corporon, held her cell phone near the microphone and played a video of her son singing the national anthem. With their hands over their hearts, many in the courtroom stood. Some sang along.

Soon afterward, Judge Thomas Kelly Ryan read his order, affirming the jury's verdict and sentencing Cross to death by lethal injection.

Cross, who represented himself during the trial and was found guilty in August , has a history of espousing anti-Semitic and white supremacist views and was thought to be targeting Jews when he launched the April 2014 attack. In September, jurors said he should get the death penalty for the killings.

Tuesday was an emotional day in court, with the prosecutor tearing up as he said the killings were the most egregious crimes the county had ever seen, CNN affiliate KSHB reported

"This is the most disruptive, evil human being I've encountered in my career," Johnson County District Attorney Steve Howe told reporters after Tuesday's hearing, according to KSHB.

"This is the most disruptive, evil human being I've encountered in my career" Steve Howe, Johnson County DA pic.twitter.com/3NvwCRnAX8 — Shannon Halligan (@ShanHalligan) November 11, 2015

The shooting rampage started when Cross killed William Lewis Corporon, 69, and his 14-year-old grandson, Reat Griffin Underwood, at a Jewish community center in a Kansas City suburb. Cross then murdered Terri LaManno, 53, at the nearby Village Shalom Retirement Community.

Lukas Losen, Underwood's brother and William Corporon's grandson, was among the family members who spoke out in court on Tuesday. The seventh-grader said he lost his childhood in a split second the day of the shooting.

"Now I've watched my grandma ... try to exist with a broken heart and live in the house alone. Some of my earliest memories after the tragedy include waking to the sound of my mother sobbing each morning, feeling a sense of denial and depression and desperately wanting my old life back," he said.

After the judge read his sentence Tuesday, Cross shouted out "Heil Hitler" multiple times in court.

"One day my spirit will rise from my grave and you all will know that I was right," he said.

Despite his apparent intentions, none of the three people Cross killed were Jewish.

JUST WATCHED 2014: Accused Kansas shooter has racist past Replay More Videos ... MUST WATCH 2014: Accused Kansas shooter has racist past 02:37

He later pleaded not guilty but admitted to the killings, claiming he "wanted to kill Jews, not people," according to CNN affiliate KCTV.

In court on Tuesday, Cross argued with the judge, KCTV reported , saying he had been demonized by the media and didn't get a fair trial.

"I can't get a fair trial because of you. This court, it is because you're under rule of the Jews, and you support yourself accordingly," Cross said.

His request for a new trial was denied, CNN affiliates reported.