WASHINGTON — His nickname was Scarface, the legacy of a brutal knife attack on a dark Houston street three decades ago that left his left cheek permanently marred. Friends and neighbors in Texas said that he could be gruff and intimidating, and that he often stood outside his house at night smoking and talking on his cellphone in a language they did not understand.

But 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.

Mr. Arbabsiar, now in custody in New York, stands accused by federal prosecutors of running a global terrorist plot that stretched from Mexico to Tehran, and that was directed by the Quds Force of Iran’s Revolutionary Guards. Many of his old friends and associates in Texas seemed stunned at the news, not merely because he was not a zealot, but because he seemed too incompetent to pull it off.

“His socks would not match,” said Tom Hosseini, a former college roommate and friend. “He was always losing his keys and his cellphone. He was not capable of carrying out this plan.”