HOUSTON—A Mexican national facing execution in Texas this week has drawn support from Mexican officials, a former Texas governor and U.S. Secretary of State John Kerry, who appealed to Gov. Rick Perry and state courts for a reprieve — so far, unsuccessfully. Edgar Tamayo, 46, a Mexican citizen, is scheduled to be put to death Wednesday for fatally shooting Houston Police Officer Guy Gaddis in 1994. Gaddis, 24, had been flagged down near a nightclub by a man who accused Tamayo of robbing him. The officer arrested Tamayo, handcuffed him and put him in the back of his patrol car....