BUFFALO — An anti-abortion extremist defending himself in a federal trial sat on the witness stand yesterday 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-year-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 the following day.

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

“You can shut down a building. You could cause a fracas, but that doesn’t stop a killing … They could go to another building,” Kopp said. “The doctor is the one essential thing. When he doesn’t show up at work, the legs of kids don’t get pulled off.”

Through more than an hour on the stand, Kopp often looked directly at jurors, holding their gaze as he answered his questions in a calm, conversational tone.

“Were you surprised to hear Mr. Slepian died? I was very grieved. I’m grieved to the present day. I don’t know what else to say. I’m also sad he was doing what he was doing,” he said.

Kopp drew a strong rebuke from U.S. District Judge Richard Arcara when he began a series of questions related to the development of a fetus, including when a heartbeat, brainwaves and fingerprints can be detected.