A Kansas judge again refused to block a confessed killer from arguing that his slaying of one of the few late-term abortion providers in the U.S. was a justified act aimed at saving unborn children.

Scott Roeder has admitted killing Wichita doctor George Tiller on May 31, 2009. He wants to present evidence that he did so in the honest belief that Tiller posed an imminent threat to unborn children and killing him would save them — a defence that could lead to a conviction on the lesser charge of voluntary manslaughter.

The prosecution has argued that that defence is invalid because there's no evidence Tiller posed an imminent threat at the time of the killing.

But the judge said during a hearing Tuesday that until the defence decides which evidence it will present, it's difficult for him to rule on its admissibility.

Some opponents of abortion were pleasantly surprised by the decision and eager to hear Roeder plead his case. Tiller's colleagues and abortion rights advocates, on the other hand, are outraged and fear the court's actions give a more than tacit approval to further acts of violence.

"This judge has basically announced a death sentence for all of us who help women," said Dr. Warren Hern of Boulder, Colo., a long-time friend of Tiller who also performs late-term abortions.

"That is the effect of the ruling."

Roeder admitted killing

The facts of the case are not in dispute: On a balmy Sunday morning, Roeder got up from a pew at Wichita's Reformation Lutheran Church at the start of services and walked to the foyer, where Tiller and a fellow usher were chatting. Wordlessly, he pressed the barrel of a .22-calibre handgun to Tiller's forehead and pulled the trigger.

Prosecutors charged Roeder with first-degree murder. The 51-year-old from Kansas City, Mo., later admitted to reporters and in a court filing that he killed Tiller. The prosecution stands ready with more than 250 prospective witnesses to prove it.

But what had been expected to be a simple trial was altered Friday when Sedgwick County Judge Warren Wilbert decided he would allow Roeder to build a defence case for a lesser charge of voluntary manslaughter.

Kansas law defines voluntary manslaughter as "an unreasonable but honest belief that circumstances existed that justified deadly force." A conviction could mean a prison sentence closer to five years, instead of a life term for first-degree murder.

Prosecutors argued Monday that such a defence should not be considered because of the lack of evidence of an imminent threat.

"The State encourages this court to not be the first to enable a defendant to justify premeditated murder because of an emotionally charged political belief," the prosecution wrote.

As events unfold inside a Wichita courtroom, the Kansas Supreme Court is also considering a challenge from four media outlets, including The Associated Press, over the judge's decision to bar reporters from witnessing jury selection.

But key questions being asked outside the courtroom have galvanized both sides of the abortion debate. Hern, the doctor in Colorado, said it's irrelevant that Wilbert won't decide whether to allow jurors to consider a conviction on the lesser charge until after Roeder's defence presents its evidence.

"The damage is done: the judge has agreed to give him a platform," Hern said. "It is an act of incomprehensible stupidity on the part of the judge, but he is carrying out the will of the people of Kansas, who are trying to get out of the 19th century."

Justice Department urged to act

The Feminist Majority Foundation also denounced the ruling, saying Wilbert essentially was allowing a justifiable homicide defence. The group urged the U.S. Justice Department to file federal charges under the Freedom of Access to Clinic Entrances Act.

Justice Department spokesman Alejandro Miyar declined to comment, citing an ongoing investigation.

Rev. Don Spitz of Chesapeake, Va., who runs a website supporting violence against abortion providers, said he and other activists from the Army of God plan to observe the court proceedings quietly.

"I am flabbergasted, but in a good way," Spitz said of the judge's decision.

Spitz acknowledged Wilbert's decision might influence some people who in the past wouldn't kill abortion providers because they risked a sentence of death or life imprisonment.

"It may increase the number of people who may be willing to take that risk," he said.