The Mad Titan perhaps even less so in the MCU), but a noble warrior. His plan has changed drastically during his history, however, and Avengers Annual #7 revealed that his original goal was much more simple and crude: destroy the sun.
Thanos has a long history in comics, spanning nearly 50 years, but his plan to murder half the universe is a recent phenomenon. It only appeared in 1991's Infinity Gauntlet, and it wasn't even Thanos' ultimate goal. His true aim was to win the heart of Death, who in the Marvel Universe is not merely a concept, but a cosmic entity. Death remained aloof and distant toward Thanos' affections, spurring the Mad Titan to gather the Infinity Stones (called gems in the comics, at least in the 90s) and murder half the universe in a grand display of power.
In Avengers Annual #7, Adam Warlock travels to Earth and warns the Avengers of Thanos' arrival. Gamora attempted to assassinate Thanos after learning his true plan, but stood no chance against her enemy and was mortally wounded just before Warlock arrived. Warlock explains Thanos' machine that can destroy stars - and if Sol (Earth's star) explodes, the resulting supernova will destroy the planet and the billions of humans who call it their home. Thanos arrives with a massive fleet, but the Avengers punch a hole through his front lines, and with an attack led by Adam Warlock, Thanos is defeated and quickly retreats.
The character of Thanos was in his infancy and wasn't yet the Machiavellian mastermind known by fans today. In Thanos Quest #1, he itted he wasn't aware of the Infinity Stones' true power - he wanted to destroy every star in the galaxy, but even that wouldn't wipe out those living in deep space or civilizations with the power to survive supernovae. With the mastery of time, space, reality, mind, soul and power, he could accomplish so much more.
The Marvel Universe has no shortage of doomsday weapons (and people) who threaten to destroy the Earth, perhaps on an issue-by-issue basis. Thanos remains one of the most deadly threats, but he has at least been stopped multiple times by the Avengers and other heroes. One can only wonder how Thanos might have faded into obscurity had he continued with his plan to destroy suns instead of trillions of beings.
