Defendant: I Killed Kansas Abortion Doc.

Updated 6:44 p.m. EST

The man accused of murdering prominent Kansas abortion provider Dr. George Tiller took the stand in his own defense Thursday and promptly admitted that he fatally shot Tiller in an an attempt to save unborn children.

Scott Roeder, who has pleaded not guilty to first-degree murder and aggravated assault, was sworn in and made no effort to deny prosecutors' claims that he killed Tiller in the foyer of Tiller's Wichita church on May 31. He previously confessed publicly to shooting Tiller.

Roeder's defense team had argued for the lesser charge of voluntary manslaughter, which is defined as an unreasonable but honest belief that deadly force was justified and could have resulted in a sentence of just five years, reports CBS News correspondent Dean Reynolds.

But late Thursday Roeder's judge ruled that the jury cannot the voluntary manslaughter charge. Judge Warren Wilbert ruled on the possible charges after the defense rested Thursday afternoon.

Legal experts had considered the reduced charge a long-shot.

"Maybe he'll feel vindicated by his ability to publicize his views on abortion," Loyola University Law School professor Jamie Carey told Reynolds.

Roeder calmly admitted today what he has never denied.

Asked by defense attorney Mark Rudy, "On May 31, 2009, did you go to the Reformation Lutheran Church and shoot and kill George Tiller?" Roeder replied simply "Yes."

Asked later by his attorney about his views on abortion, the 51-year-old Roeder said he believes it amounts to murder.

"From conception forward it is murder. ... It is not a man's job to take life. It is our heavenly father's. He is our creator," said Roeder, who is from Kansas City, Mo.

Roeder replied "yes" or "no" to many of the questions he was asked, and his attempts to elaborate drew frequent objections from prosecutors, who say Roeder lacks the medical expertise to describe Tiller's practice.

When asked to detail the types of abortion procedures he was familiar with, Roeder answered "four or five" and then listed them. In one instance, he described a procedure as the fetus being "torn limb from limb" - a characterization that prompted a quick objection from the prosecution.

Before Roeder took the stand, Judge Wilbert barred Kansas' anti-abortion crusading former attorney general Phill Kline from testifying after listening to a preview of Kline's testimony without the jury present.

Kline investigated Tiller's clinic, Women's Health Care Services, in 2006 because he suspected Tiller was violating Kansas' laws pertaining to late-term abortion. The case was later dropped because of jurisdictional issues.

Wilbert said allowing Kline's testimony would be "inappropriate" at the trial, and said much of it amounted to "exactly what this court seeks to avoid."

"I said I would not allow this courtroom to turn into a forum or a referendum on abortion," Wilbert said.

The decision to bar Kline's testimony hampers the strategy of the defense, which had hoped to show Roeder based his actions in part on Kline's belief that Tiller was breaking the law - a potential step toward a lesser conviction of voluntary manslaughter.

Voluntary manslaughter in Kansas is defined as "an unreasonable but honest belief that circumstances existed that justified deadly force." A conviction on the lesser charge involves considerably less prison time.

Wilbert reminded Roeder's attorneys in court Wednesday that they must couple a voluntary manslaughter defense with a showing of imminent danger posed by the doctor. He noted abortion is legal in Kansas.

Wilbert will rule later on whether to allow jurors to consider the lesser charge.

The two counts of aggravated assault Roeder faces are for allegedly using a gun to threaten two ushers who tried to stop him after the May 31 shooting at Tiller's Wichita church.