WARNING: The following contains spoilers for Marvel's Avengers: Infinity War, in theaters now.
Six years after his introduction in the mid-credits of The Avengers, Marvel's Avengers: Infinity War finally provides fans with an idea of who Thanos is and what his motivations are. Within two and a half hours, directors Joe and Anthony Russo brothers tell the Mad Titan's tale, and make audiences feel sympathy for this cosmic death dealer. As we've come to expect from a Marvel Studios film, this isn't a direct adaptation of the classic villain from comics. While the two versions are similar in many respects, they're quite different in others.
Thanos made his Marvel Comics debut in 1973 in Iron Man #55, by Jim Starlin and Mike Friedrich. Since then Starlin has maintained a sense of ownership over the character, writing a number of miniseries and graphic novels starring his best-known creation, including Thanos Quest, The Infinity Gauntlet, Infinity War and Infinity Crusade. These are the storylines that gave us the Infinity Gems (referred to as Infinity Stones in the Marvel Cinematic Universe), and the Infinity Gauntlet.
His initial appearance and Starlin’s early-1970s run on Captain Marvel establish that he's a member of the Eternal race from Titan, one of Saturn’s moons. Further, we learn of his familial ties, as these issues also introduce his father A’Lars, aka Mentor, the leader of Titanian society, his mother Sui-San, his brother Eros and his ancient grandfather Kronos. It was in Captain Marvel #26 (1973) that Starlin introduced the personification of Death, who stands with Thanos as he declares: “By the might of the legions I command -- and recognizing Death as my only comrade, I seized power on my homeworld Titan.” From this point on, the idea that the Mad Titan kills in the name of Death herself has become a cornerstone of the character.
It wasn’t until 2013’s Thanos Rising miniseries by writer Jason Aaron and artist Simon Bianchi that we became privy to the details of the tyrant’s birth and childhood. It's revealed his skin color and bad complexion are due to a mutation called the Deviant Syndrome. While his father loved him all the same, when his mother first laid eyes on him, she tried to kill him. That early trauma began his obsession with death. Through most of his young life he was friends with a girl with distinct facial markings who encouraged him to find his true self. He eventually came to understand she is the embodiment of entropy, and realized Death has been his companion all along.