LOS ANGELES—A frail-looking Robert Durst struggled to hear his attorney as he took the stand at his murder trial—in a wheelchair, not a witness chair—and denied killing his best friend two decades ago. Durst, 78, looked extremely feeble and strained to speak Monday as his attorney, Dick DeGuerin, took the risky move of calling a murder defendant to testify. “Bob, did you kill Susan Berman?” DeGuerin said to open the testimony of the New York real estate heir. “No,” Durst answered. “Do you know who did?” DeGuerin asked. “No, I do not,” answered Durst, who was expected to return to the stand on Wednesday. Berman acted as Durst’s de facto spokesman after the 1982 disappearance of his wife, Kathie, who was later declared dead, though no body was found. However, prosecutors contend that Durst shot Berman in the back of the head in her Los Angeles home in 2000 because …