Even the biggest villains eventually fall in the Marvel Universe and few defeats were as gruesome as the time Thanos was brutally killed by Doctor Doom in the Secret Wars event. The Mad Titan's arrogance, thirst for power, and lack of respect for his powerful opponent ultimately led to his demise, as Doom ripped his skeleton from his body, killing Thanos in a matter of seconds.
2015's Secret Wars event was a spiritual successor to the iconic Marvel Super Heroes Secret Wars event. The comic saw Marvel's multiverse destroyed with the only remaining universes left being the main Earth-616 as well as Earth-1610. The surviving heroes were transported to Battleword which contained the remaining pieces of universes that had been destroyed. Ruling Battleword was Doctor Doom - who with an upgrade from the Beyonders, ruled the planet like a god. One villain who wasn't impressed with Doom's reign was Thanos, who made the mistake of confronting the longtime Fantastic Four villain and not accepting his offer of mercy.
In Secret Wars #8 by Jonathan Hickman, Esad Ribic, Ive Svorcina, Chris Eliopoulos, and Idette Winecoor, Thanos made it past Doom's defenses and immediately challenged him. As Doom offers him the chance to keep his horde in exchange for his loyalty, Thanos scoffs at the ruler of Battleworld. He tells Doom that he was a god when he had the Infinity Gaunlet and he chose not to use the powers as a "mortal" as he would have. The Mad Titan said he wielded in the judgment of all living things while taking a pot-shot at Doom, calling him a "weak god" and a "pretender." Thanos even tells Doom he should bow before him. When Doom asks if he has the gauntlet on him and is told no, he rips Thanos' spine out of his body, instantly killing him.
While Thanos would eventually be brought back to life after the multiverse was restored at the end of Secret Wars, his death at Doom's hands is one of the most brutal kills in Marvel Comics' history - and was understandably way too dark to ever adapt to the Marvel Cinematic Universe.
In the scene, Thanos overestimated his own power and underestimated Doom's. Considering that Doom was effectively a god at the moment, trying to talk down to him was a huge mistake. Thanos is also lucky the multiverse eventually restored itself because getting so brutally killed by Doom would have been an embarrassing way to die. The next time these two supervillains meet (even in a normal, non-Battleworld setting) Thanos should probably pay some respect to Doctor Doom, as he's really his equal or more when it comes to pure power.