(CNN) -- A Kansas jury deliberated just 37 minutes before convicting an anti-abortion activist of first-degree murder in the fatal shooting of an abortion provider.

The jury found Scott Roeder, 51, guilty of gunning down Dr. George Tiller, who operated a clinic in Wichita where late-term abortions were performed. Roeder, 51, faces life in prison when he is sentenced on March 9.

Tiller's family said the jury reached a "just" verdict.

"At this time we hope that George can be remembered for his legacy of service to women, the help he provided for those who needed it and the love and happiness he provided us as a husband, father and grandfather," the family said in a written statement.

A day earlier, Roeder told jurors he had shot Tiller in the foyer of Reformation Lutheran Church as Sunday services began. Testifying as his only defense witness, he said he believed he had to kill Tiller to save lives. He said he had no regrets.

"There was nothing being done, and the legal process had been exhausted, and these babies were dying every day," Roeder said. "I felt that if someone did not do something, he was going to continue."

"His testimony was delivered very matter-of-factly, but its contents were chillingly horrific," prosecutor Ann Swengel said in her closing argument. "He carried out a planned assassination, and there can be no other verdict in this case ... other than guilty."

Quick verdict surprised prosecutor

Prosecutors initially fought to keep abortion out of the trial, claiming that Tiller's death was a straightforward case of premeditated murder.

Eventually, the abortion issue took center stage as prosecutors portrayed Tiller as a target of Roeder's anti-abortion agenda, and defense lawyers attempted to mitigate his culpability under the theory that he believed Tiller's death was justified to save the lives of others.

Defense attorney Mark Rudy told jurors in his closing argument that Roeder "thought that the babies kept on dying" and he had to stop Tiller from "killing more babies."

Another defense attorney, Steve Osburn, said Roeder was "disappointed," with the verdict. But he added, "He's known that this day was going to come, I think."

Osburn said his client "feels remorse toward the family, but not for what he did."

The trial drew activists from both sides of the abortion debate to the courtroom, and a van plastered with slogans and photographs of fetuses was parked in a prominent spot in front of the courthouse.

Among the attendees were the Rev. Michael Bray, whose history in the anti-abortion movement includes 1985 conspiracy convictions in connection with a string of clinic bombings, and Katherine Spillar, executive vice president of the Feminist Majority Foundation.

Congregants from Reformation Lutheran testified that they had seen Roeder at church several times before the day he killed Tiller by shooting him at point-blank range in the head.

Jurors heard emotional testimony from church-goers who rushed to Tiller's side and attempted to administer mouth-to-mouth resuscitation as he lay in a pool of blood. Others, meanwhile, followed Roeder into the church parking lot, where he threatened to shoot them.

Roeder also was convicted of aggravated assault in connection with threats he made to two ushers, Gary Hoepner and Keith Martin.

As Roeder pulled away in his car, Martin testified, something moved him to throw the coffee cup he was holding at the vehicle. "Frustration, I guess, lack of accomplishment, nothing else to do."

Prosecutors also called employees of the pawn shop where Roeder purchased the .22-caliber Taurus pistol believed to have been used to shoot Tiller. The gun was never found, but surveillance video and receipts showed that he purchased the gun on May 18 and received it on May 23, the week before he shot Tiller.

Roeder's defense team did not dispute much of the factual evidence. Roeder testified that he chose to target Tiller at church because it presented the best "window of opportunity" to attack Tiller, who traveled in an armored vehicle and whose clinic was a "fortress."

He admitted bringing the pistol with him to Lutheran Reformation on May 24 with the intention of shooting Tiller, but the physician did not attend services that day. So, Roeder testified, he returned the following week.

"Do you feel as though you've successfully completed your mission?" Sedgwick County District Attorney Nola Foulston asked Roeder on Thursday.

"He's been stopped," Roeder answered.

His testimony was intended just as much for the jury as it was to convince Judge Warren Wilbert that evidence existed to support a possible conviction of voluntary manslaughter. A conviction on the lesser offense, which is defined as "an unreasonable but honest belief that circumstances existed that justified deadly force," would have set Roeder free from prison after five years.

Earlier in the trial, Wilbert said he would rule after hearing evidence in the case, acknowledging that he felt the defense faced "an uphill battle." Ultimately, he rejected the theory, saying testimony did not support the defense claim that Roeder's beliefs justified using deadly force against Tiller.

"There is no imminence of danger on a Sunday morning in the back of a church, let alone any unlawful conduct, given that what Tiller did at his clinic Monday through Friday is lawful in Kansas," the judge said.

In Session's Lena Jakobsson and CNN's Emanuella Grinberg contributed to this story.