Judge Ken Anderson of Williamson County, facing criminal charges over prosecuting a man who spent 25 years in prison for a killing he did not commit, resigned Tuesday as he awaits trial over whether he concealed evidence as a district attorney. Mr. Anderson prosecuted Michael Morton in 1987 for murder in his wife’s slaying. Mr. Morton was exonerated in 2011 based on DNA evidence. In April, a special court of inquiry determined that Mr. Anderson concealed evidence favorable to the defense.