At first, the slaying of Shahabuddin Rana, 44, inside his Pizza Mart in the 2300 block of Fourth Street NE appeared to have stemmed from a conventional robbery, police said. But they said a bizarre story took shape in the days afterward, leading to the arrest Friday of Shanika Robinson, 26, of Capitol Heights.In an affidavit filed in D.C. Superior Court, Detective Robert Cephas said a witness told him that Rana had been paying Robinson to continue in a sham marriage with his brother, a Pakistani national who helps run the Pizza Mart in the Edgewood neighborhood. The scheme was intended to improve the brother's immigration status and help him eventually get his green card, the affidavit says.The deal fell apart, however, after the brother, whom police have not publicly identified, found out that Robinson "was having sexual affairs," Cephas wrote. Even though the brother and Robinson were married only on paper and did not live together, Rana "became upset" with Robinson and told her "that she would not be getting paid the money that they agreed upon anymore," the affidavit says.Robinson, accompanied by three men, allegedly confronted Rana while he was alone in the 24-hour pizzeria late Aug. 17 or early Aug. 18. Police said his assailants slashed and beat Rana, fracturing his skull in several places, then tried to burn his body before fleeing with about $2,000 in cash and packages of Newport cigarettes and Dutch Masters and Black and Mild cigars.