Thanos lusted after power, acquiring more than any of the other Eternals. Since childhood, he had a preoccupation with death. During his travels across the universe, he actually meets Death, who takes the form of a woman. In the movie, the Chitauri leader tells Thanos that to invade Earth with the Avengers would be “courting Death itself.”
That makes Thanos smile. Courting Death is exactly what he’s doing.
Thanos became infatuated with Death, which led to the first Thanos war and his quest to please Death by killing everyone in the universe.
A bit over the top? Tell it to Thanos.
Thanos gathers an army of the worst aliens he can find and starts a multi-galaxy killing spree, starting with his home planet of Titan. After murdering millions there by nuclear bombardment, he goes after the Cosmic Cube.
Here’s where the Avengers enter the picture.
It’s possible the First Thanos War, or some variant of the story, will be the basis of the future Avengers movie. In the comics, Thanos comes to Earth in search of the Cosmic Cube. That’s the shiny box featured so heavily in the previous movies. It’s the object called the “Tesseract.”
Thanos wants to use the power of the cube to kill enough life to impress Death.
In his way stand the Avengers. Hopefully the movie will bring another character from the comics to life and include the Kree warrior Captain Mar-vell in the sequel. It was Captain Mar-vell, after all, that helped defeat Thanos and prevent the death of the universe.
Thanos wasn’t destroyed though. He returns over and over again, each time seeking the power needed to impress Death.