When the cosmic conqueror Thanos got his hands on the all-powerful Infinity Gauntlet, Marvel Comic history was made. The comic book story that inspired both Avengers: Infinity War and Avengers: Endgame may not have been as massive a crossover spectacle on the page, but the comic was no less shocking, or brutal, with how Thanos dispatched every single hero in the Marvel Universe. But nobody got it worse than the leader of the X-Men, Cyclops.
The majority of the murders actually come after Thanos performs his infamous "snap," wiping out half of all life in the universe, including most of Earth's Mightiest Heroes. While his action doesn't impress Mistress Death, it DOES attract the attention of just about every major hero. With Captain America leading the fight, the heroes who survived the snap are transported to deep space, where a power-crazed Thanos stands ready to prove his almighty power. To Avengers, Heralds, and X-Men alike.
Hoping to once again impress Mistress Death with his fighting prowess, Thanos allows himself a .05% chance of being defeated by the oncoming heroes. Although they are aware that defeat is a near certainty, the superheroes stage their attack against Thanos, with Warlock and Silver Surfer observing a light-year from the field. In Infinity Gauntlet #4, it all comes to a head. The fight goes as expected, with Thanos killing each hero one by one. Iron Man is decapitated; Spider-Man is bludgeoned to death; Thor and Drax the Destroyer together hold their own for a bit before each falling. But Cyclops suffers one of the worst, slowest deaths imaginable.
At the moment he uses his optic blasts, Thanos conjures an airtight, unbreakable cube around his entire head. Unable to use his blasts to free himself, he collapses onto the ground, as Captain America unsuccessfully tries to use his shield to crack the cube open. Cyclops lingers for a bit before collapsing onto the ground, yet another casualty in a futile attempt to stop the Mad Titan.
While every other hero gets a relatively quick death, it's the way Cyclops lingers for a while before ultimately succumbing that makes his a particularly brutal death. Even when Captain America is the only hero left standing, Thanos quickly dispatches him with a seemingly nonchalant smack which breaks his neck. Sure, eventually the universe is restored, along with every hero and life that was lost, but watching every superhero fall one by one is an emotional sight. And while the rest of the X-Men were killed off as well, it was still a shock to the system just how callously and cruelly one of Marvels' leading mutants fell.