BUFFALO, N.Y. - An anti-abortion extremist defending himself in a federal trial sat on the witness stand Monday to explain why he killed a doctor.

"I don't like killing. I don't like the thought of it,'' James Kopp said as he asked and answered a series of questions from his seat next to the judge. "The plan was to keep everyone alive at the end of the day.''

Kopp, 52, charged with violating the federal Freedom of Access to Clinic Entrances Act by killing Dr. Barnett Slepian in 1998, is serving a 25-years-to-life sentence for a state conviction on second-degree murder in the doctor's death.

He has acknowledged firing the shot that killed Slepian inside his Amherst home but said his goal was to incapacitate -- not kill -- the doctor to prevent him from performing abortions.

Kopp, who estimated he had participated in more than 100 anti-abortion demonstrations outside clinics, said he had become convinced the best way to prevent abortion would be to gravely injure an abortion provider.

"The doctor is the one essential thing,'' he said. "When he doesn't show up at work, the legs of kids don't get pulled off.''

Kopp compared abortion opponents to people who tried to kill Adolf Hitler and tried to protect Jews from the Nazis during the Holocaust. The tactic drew objections from prosecutors and warnings from the judge.

Earlier Monday, he called three witnesses, all of them fellow abortion opponents who testified they never knew Kopp to be violent.

Kopp was also a suspect in four non-fatal shootings of doctors in the Hamilton, Winnipeg and Vancouver areas and in Rochester, N.Y., in the decade before Slepian's death.