FORT WORTH (CBSDFW.COM)After taking just 45 minutes on Monday to find Thomas Lester Harper guilty of murder for shooting an 18-year-old Good Samaritan, a Tarrant County jury took even less time to sentence the man to life in prison.

Harper was found guilty of murdering Clarence Robinson in December of 2011. Minutes before shooting the teenager Harper had been the driver in a chase through Arlington that caused a multi-car crash and killed another driver, Najee Nasir.

Robinson had approached Harper’s wrecked SUV, trying to rescue the man’s two screaming children. Officials with the Tarrant County District Attorney’s Office say as Robinson attempted to free one of the children from the wreckage Harper pointed a gun at him and said, “This is going to be your bad day,” and shot the teenager in the head.

Immediately after the shooting a homeland security officer and a constable arrived at the scene and held Harper at gunpoint until Arlington police arrived.

“If this defendant had not stolen Clarence Robinson’s life, he would have turned 21 last Saturday,” said Prosecutor Amy Collum. “We hope this verdict and sentence gives Robinson’s family, as well as Najee Nasir’s family, some measure of peace.”

During the trial the defense claimed that Harper was acting in self-defense. Harper’s attorney, Ezekiel Tyson, told the court, “He didn’t think Mr. Robinson was trying to help him. His intent was not to harm Mr. Robinson. He was trying to protect his children.”

The jury sided with the prosecution.

During the punishment phase of the trial, prosecutors presented evidence that in years past Harper had once pulled a gun on a teenage girl with whom he fathered a child and had also once pulled a gun on the girl’s father. Apparently trying to unravel Harper’s self-defense claim, a college student who witnessed the 2011 deadly wreck also testified for the prosecution. The student told the court that as Harper was led away in handcuffs, he heard him say, “I would f—ing do it again.”

After finding Harper guilty on Monday the jury took just 40 minutes one day later to sentence the 30-year-old to the maximum sentence of life in prison, plus a $10,000 fine.

“The horror the defendant has inflicted on these families—the Robinson’s, the Nasir’s and not least his own— is almost incomprehensible,” said Prosecutor Jack Strickland. “It is all the more so because it was so senseless and random.”

Harper must serve at least 30 years behind bars before he is eligible for parole.

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