The following contains spoilers for Sins of Sinister #1, now on sale from Marvel Comics
The X-Men have a lot of power under their belts, especially when one takes Omega-Level Mutants like Storm, Magneto, and Legion into consideration. The fusion of their abilities can create wonders, reshape entire worlds -- and if utilized in particular means, kill pretty much anyone in the Marvel Universe.
Sins of Sinister #1 (by Kieron Gillen, Lucas Werneck, Geoffrey Shaw, and others) reveals a number of grim mutant weapons and advancements, made possible by Krakoa and the X-Men falling under the corrupting control of Mister Sinister. Among these creations is a way to eliminate Thanos once and for all. The problem is, the attack utilizes Juggernaut in a horrifying way that also removes him from the future entirely.
In Sins of Sinister, the X-Men Sacrifice Juggernaut To Destroy Thanos
One of the big twists of Sins of Sinister is the revelation that the X-Men aren't just caught up in the plans of the villainous Mister Sinister, but actually fundamental pieces of his scheme. Thanks to the death and resurrection of Hope Summers, Sinister slipped pieces of himself into all the genetic material provided for the Five. This, in turn, corrupts all of the typically noble mutant leaders of Krakoa into a far more ruthless and cruel form. This sets the stage for the Sinister-dominated future teased in Sins of Sinister, where the X-Men's mutant technologies are employed in increasingly vicious ways to ensure the domination of a Sinister-aligned Marvel Universe.
One of the first showcases of this power comes at the expense of Thanos, who quickly finds himself targeted by Sinister. Learning that Thanos is set to take another swing at wiping out the Earth, Sinister has Forge construct a temporal cannon that can fire an object at an unstoppable rate. He then has the telepaths of Krakoa manipulate Juggernaut into volunteering to be the object in question. Shrunk down to the size of a bullet and fired at the Mad Titan with Domino-assisted good luck, Juggernaut is used to effectively head-shot Thanos. This seemingly kills the villain, as a new X-Men's offensive against the Eternals allows them to lock down any potential resurrection for the immortals. However, Thanos' death also removes Juggernaut from the board, as he reenters space at such a rate and speed that no one can recover him.
The X-Men's Mutant Technologies Could Wipe Out Virtually Any Threat
Juggernaut is one of Marvel's most powerful forces, strong enough to hold the Hulk at bay on a bad day. He's repeatedly proven that he's unstoppable when properly applied to a situation, and being turned into an unstoppable bullet is the most literal way to utilize those abilities. Thanos has a habit of always crawling his way back from the limits of death, but by countering the Eternal's technology and killing him in such a way that regular healing would fail to repair, the mutants may have found the ultimate way to destroy Thanos once and for all.
It's a great showcase for how dangerous Mutant Technology can really be, especially when applied with a ruthless mindset. Since the term was coined during the beginning stages of the Krakoa Era, the concept of Mutant Technologies has allowed the mutants to transform planets and master resurrection. But if they were a little harsher about what they did, concepts like the Juggernaut bullet could be used to more or less wipe out anyone and everyone that gets in their way. It's just a handful of moral lines that have always kept someone like Xavier from doing something more damaging to the world than just preventing nuclear war.
Sins of Sinister raises the stakes of its event by confirming how dangerous Marvel's mutants can be when they don't have any moral limitations on their actions. If they were unconcerned with the cost of lives or the destructive capability they've mastered, they could turn Marvel Universe-wide threats like Thanos into an afterthought. It's also -- in some small way -- a hint that Juggernaut is strong enough to plow right through even someone as strong as Thanos. It just means he might never get the chance to stop, costing the X-Men one of their strongest physical figures.