LOS ANGELES—The estranged brother of Robert Durst, the real estate heir on trial in his best friend’s slaying, reluctantly testified Monday that the two never got along and he feared his oldest sibling would kill him. “He’d like to murder me,” Douglas Durst bluntly told jurors in Los Angeles County Superior Court. Douglas Durst, head of one of New York’s largest commercial real estate firms, said his brother was angry and bitter over an acrimonious inheritance settlement for tens of millions of dollars. He had not seen his brother in 20 years but worries because of threats he has made. The chairman of the Durst Organization, which owns some of Manhattan’s premier skyscrapers and 2,500 apartments, said he and his brother have fought since childhood. “He treated me miserably,” Douglas Durst said. “He would fight with me at every chance. He would embarrass me.” Despite the bad blood, Douglas Durst …