It can be argued that Assef is most like Rasheed in A Thousand Splendid Suns. Assef's equivalent in The Thorn Birds, however, would be a combination of Mary Carson and Luke O'Neill. Let me explain.
Like Assef, Rasheed in A Thousand Splendid Suns harbors a violent streak. In The Kite Runner, Assef is guilty of rape, murder, and theft. Although Rasheed does not kill anyone, he comes close to it. Without Mariam's intervention, Rasheed would have choked Laila to death. Both Rasheed and Assef are capable of great violence. The only difference between the men is that Assef has committed actual murders.
When we turn to The Thorn Birds, the closest equivalent to Assef (based on the capacity for violence) is Luke O'Neill. Luke does not kill anyone, but he enjoys his wedding day privileges at Meggie's expense. It can be argued that Luke actually raped Meggie during their first sexual encounter. The text is clear that Luke demonstrated little regard for his bride's sexual inexperience and made no effort to spare her physical agony:
She fought like one possessed to be rid of that ghastly, painful thing, but his weight pinned her down and his hand deadened her cries, the agony went on and on. Utterly dry because he hadn't roused her, the even drier condom scraped and rasped her tissues as he worked himself in and out, faster and faster. . . then some change stilled him, made him shudder, swallow hard. The pain dulled to raw soreness and he mercifully rolled off her to lie on his back.
Here, Luke's actions mirror that of Assef's in The Kite Runner. In his youth, Assef raped the helpless Hassan and as an adult, he makes Sohrab (Hassan's son) his sexual slave. Assef also participates in the murder of thousands of Hazaras and religious minorities. Rasheed in A Thousand Splendid Suns comes closest to Assef in terms of pure malice and savagery. Consider how Rasheed treats Mariam during their first sexual encounter after their wedding:
He slid under the blanket beside her. She could feel his hand working at his belt, at the drawstring of her trousers. Her own hands clenched the sheets in fistfuls. . . . The pain was sudden and astonishing. Her eyes sprang open. She sucked air through her teeth and bit on the knuckle of her thumb. She slung her free arm over Rasheed's back and her fingers dug at his shirt.
To get back to The Thorn Birds, Luke O'Neill is similar to Assef in the sense that both men are self-absorbed, violent, and reckless. However, Luke is not a sadist or a sociopath, at least not in the typical sense. On the other hand, Assef is openly malicious and sadistic in nature. He revels in forcing his fellow Muslims to watch his brutal executions of religious minorities or other Muslims he deems "unrighteous." Assef even gloats in Sohrab's sexual slavery and openly taunts Amir when the latter begs for Sohrab to be released into his custody. Based on his sadistic nature, Assef's equivalent is Mary Carson in The Thorn Birds.
Knowing that her sexual passion for Father Ralph de Bricassart will never be reciprocated, Mary Carson leaves the priest with a terrible choice upon her death: he can either leverage Drogheda and the 13 million pounds she bequeaths him to fulfill his ambitions in the Catholic Church or deprive Meggie (the woman he loves) of her inheritance. Mary understood the immense emotional agony Ralph would suffer in making his choice, and she delighted in that knowledge. Mary's sadistic nature is equivalent to Assef's nature: both derive actual delight from being the cause of another's suffering.