The killer who crept up behind an imam and his friend near a Queens mosque and shot both men in the head may have been settling a score in a feud between Muslims and Hispanics, police sources said early Monday. And he may have left behind another victim who unwittingly held the clue that solved the crime, the sources said. The suspected killer was taken into custody Sunday night but had not been charged or identified, according to the sources. Police officials would not confirm whether a suspect was nabbed.