The Titans are a race of beings from the planet Titan who were all but destroyed by a catastrophic event. The Titan Thanos was born on the planet Titan, and due to his severe deformities, was considered an anomaly by the Titan society and grew up as an outcast. Despite so, Thanos loved his people nonetheless.
Due to overpopulation, the planet began to run out of resources. Thanos offered a solution, to randomly kill off half of the planet's inhabitants so that the other half could truly live without having to fight each other.
The Titans saw his solution to be madness and refused to let Thanos intervene. This led to Titan eventually being devastated when the people finally snapped and turned on each other for survival, and left mostly uninhabitable. The Titans themselves went nearly extinct after the conflict.
Thanos began to believe that other planets would suffer the same fate, causing him to become obsessed with saving the universe, by killing half of each planet's inhabitants.