Mansour J. Arbabsiar, 56, the man at the center of an alleged Iranian plot to kill a Saudi diplomat in Washington, seems to have been more a stumbling opportunist than a calculating killer. Over the 30-odd years he lived in Texas, he left a string of failed businesses and angry creditors in his wake, and an embittered ex-wife who sought a protective order against him. He was perennially disheveled, friends and acquaintances said, and hopelessly disorganized.