Again, you're missing the point. Williams was clearly guilty. He had a full jury trial and over 20 years of appeals. The system isn't perfect but it's the best we have. If you're argument is that he should not be executed because the death penalty is morally wrong and can result in an irreversible mistake then that's fine. If, however, you are suggesting that Williams is innocent then your argument falters significantly. Williams had a full trial, full state court appeals and full federal court appeals. His case has been combed over in incredible detail. He's as guilty as anyone who as ever been found guilty. Moreover, the man did not reform. Even the Texas example your pointed out related to the defendant's IQ and not his guilt or innocence.
There are hundreds of thousands of trials in the US every year. For the most part (that is, a significantly high percentage) the trials are fair and impartial. There are always exceptions, however, but they tend to be so small as to be just that, exceptions. To be sure, when it comes to the death penalty there is no margin for error which is a very good reason to abolish the death penalty.
While you may not know that Williams is guilty a jury of his peers felt that he was. Five additional courts felt that he was. Far be it for me to question your grasp of jurisprudence but I think at some point we have to defer to the trier of fact who got to see all of the evidence presented at trial and appeal.
Lastly, please don't use bold when responding. It's poor 'net etiquette.