You probably haven't heard of Chris Mapp, a South Texas boat salesman and one of the six also-rans challenging John Cornyn in the Republican primary for U.S. Senate, and here's hoping you never have occasion to hear of him again after March 4. If you do, chances are disconcertingly high that it will somehow involve a smoking rifle and a dead Mexican. That would at least be in line with what he told the center-right editorial board of The Dallas Morning News, the most influential newspaper in the state: South Texas businessman Chris Mapp, 53, told this editorial board that...