James C. Kopp's new attorney has not ruled out the possibility of the accused killer of Dr. Barnett A. Slepian taking the witness stand when he goes to trial next year. "I can't rule that out," said Bruce A. Barket, a Long Island attorney and abortion opponent who was approved Tuesday to head Kopp's defense team at Erie County Court. "That hasn't been decided yet." If he does testify in his own defense, would Kopp reveal what he was doing - and what his motivations were - on the night Slepian was killed? Barket declined to speculate on defense strategy,...