Yes, Clinton executed a mentally challenged man (who was a minor when the actual crime took place) but so did George W. Bush. As a matter of fact, Bush actually killed at least 5 retarded people, including one Oliver D. Cruz. Bush repeatedly has stated that Texas does not execute the mentally challenged, however he opposed a bill that would have banned the practice.
Bush, a so called “born again Christian” also executed a woman in Texas, Karla Faye Tucker, who was also a “born again Christian” and devoted her life to god while in prison. Bush in an interview after her execution, reportedly mocked the woman’s plead for mercy just days before begging for clemency on Larry King Live.