Despite their insistence to the contrary, there's a distinct possibility the Eternals of the Marvel Cinematic Universe bear much more responsibility for the actions of Thanos than they claim. In comics, Thanos was spawned by the Eternals. The parents of Thanos were the Eternals Sui-San and A'Lars, and their actions play direct roles in their son's transformation into one of the deadliest threats in the entire universe.
A'Lars and Sui-San shaped the monster the Mad Titan would one day become in conflicting but equally important ways, as would the Eternals' shared lineage with their bestial enemies, the Deviants.
Who Were Thanos' Parents, A'Lars (aka Mentor) and Sui-San?
Born into the ruling family of the ancient Eternals in the wake of a devastating civil war, A'Lars' aversion to conflict was so great that it drove him to abdicate the throne to his brother, Zuras, and exile himself to the distant moon of Titan. Taking on the moniker of "Mentor," A'Lars dedicated his impressive scientific mind to improving the universe. It was during this endeavor that A'Lars met Sui-San, the last survivor of the war-mongering Eternals his father had banished from the Earth in the aftermath of the war.
Together, A'Lars and Sui-San transformed the barren moon of Titan into a lush, life-sustaining paradise and re-populated the planet with a new generation of Eternals through the use of advanced technology. Over the course of their work, A'Lars and Sui-San fell deeply in love and eventually settled down in the hopes of starting a family of their own. Their union produced twin sons; the average-looking Eros and the heavily deformed Thanos, whose physical mutation is eventually linked to lingering bits of Deviant DNA lurking within the Eternals.
While A'Lars took Thanos' abnormal appearance in stride, Sui-San was driven insane by the mere sight of her son. Convinced the child would bring ruin to the universe, Sui-San attempted to kill Thanos moments after his birth. This traumatic experience shaped Thanos' perception of life forever, despite A'Lars' best attempts to provide both of his sons with a happy upbringing in the absence of their mother, who was institutionalized.
Why Did Thanos Kill A'Lars and Sui-San?
Sui-San was eventually reunited with Thanos, but the nature of her relationship with her son, and what ultimately happened to her, varies between retellings of Thanos' backstory. In most versions, Sui-San maintained a cold relationship with Thanos, while in others she tried to make amends by treating her son better. Regardless of her attitude toward him, Thanos' killed her in every continuity, either indirectly during an assault on Titan or by personally dissecting her alive in an attempt to find out why he is so different from everyone else.
Upon being confronted with the undeniable truth of Thanos' evil, A'Lars spent the rest of his life undermining his son's reign of terror. A'Lars assisted many heroes in their battles against Thanos with his skills as a scientist, acting as a benefactor to Drax the Destroyer, the Silver Surfer and Captain Mar-Vell. Despite his heroic efforts, A'Lars ultimately met his end when Thanos came before him infected with a terminal illness to ask for aid. After explaining that it would be impossible to create a cure in the short time that Thanos had left, A'Lars makes it clear that he would not help his son even if it was possible. Thanos' response is as violently efficient as it is personal.
Thanos' parentage within the MCU has been limited to a single reference by the Red Skull, but The Eternals does leave a window open for an appearance by one or both of his parents. No connection between the Eternals and the Titans has been established as of yet, but there may already be hints of one. After all, Thanos' appearance does bear some similarities to that of Kro, the humanoid Deviant featured in The Eternals' most recent trailer.
Given the Deviants' seemingly innate drive towards destruction, a link between them and Thanos would certainly explain a few things about the Titan's mindset. Perhaps even more enticingly, it could signify a direct, almost poetic connection between the creators of the universe and the man who came close to destroying everything with a literal snap of his fingers.