José Padilla’s conversion to Islam could have helped him leave behind a life of violence and thuggery that saw him in and out of jails. Instead, his new religion and his old ways apparently combined to make him the perfect candidate for terrorism, according to prosecutors who say the former South Floridian, 31, plotted with al-Qaida to build a radioactive bomb that could have rained nuclear material over city blocks. A decade ago, he was planning a new life for himself and his future wife. Both converted to Islam and Padilla took it very seriously, asking questions and attending local...