Thanos has a reputation as Marvel's ultimate villain. The Mad Titan has earned that appellation from his many schemes to get ultimate power and use them to destroy as many lives as possible. Thanos's obsession with Mistress Death, the personification of death, appeared to be the motivation behind his actions, but that's a very simplistic way of looking at the character. Thanos is actually complex, something imbued in the villain by his creator, writer/artist Jim Starlin.
Source：https://www.cbr.com/thanos-marvel-comics-history-vs-mcu-legacy/
