WARNING: The following article contains spoilers for Hellions #6, by Zeb Wells, Carmen Carnero, David Curiel, VC's Ariana Maher, and Tom Muller, on sale now.
The Hellions' mission to end the lethal X of Swords tournament before it started was always doomed to fail. The entire team was wiped out during the course of their mission to steal the swords of Arakko's chosen fighters. The mission went sideways when the team entered Arakko and battled its twisted mutants, the Locus Vile.
While that thinned their numbers, half of the team made it back to Krakoa in Hellions #6. Once they returned home, their suicide mission turned into a massacre, perpetrated by the man behind the most infamous mass murder spree in X-Men history, Mister Sinister.
The Locus Vile brutally took out Wild Child, Nanny, Orphan-Maker, and seemingly Sinister himself in Arakko. Empath deserted the team and headed for the gateway back to Avalon as things got even worse. Psylocke, Greycrow and Havok (whose hands were lopped off by a mutant named Hex Butcher) were able to escape through the gateway with the time Wild Child bought them.
Instead of taking one of the swords with them, the surviving members of the team carried DNA samples of the Vile Locus. Sinister's desire to add new samples to his collection of mutant DNA was the reason for the mission all along. He convinced an irate Psylocke to deliver the DNA to Krakoa by referencing a deal between them, one of many he's made lately.
When they make it through the gate, after Greycrow gets revenge on Empath for enslaving him, the surviving Hellions were ambushed by Sinister. Using a smokescreen to avoid being caught on camera, Sinister was able to pick off Psylocke, Havok, and Greycrow. After Empath succumbs to his wounds, Sinister rips his clothes to look like he's also been attacked and cries for help.
Sinister isn't a stranger to massacres. He spearheaded one in his first appearance and was the mastermind behind the events of the very first X-Men crossover, "Mutant Massacre," which certainly lived up to its name.
Gambit recruited and guided Sinister's Marauders through the sewers of New York. Once there, they slaughtered the sewer-dwelling Morlocks, drawing them into conflict with all three of the X-Men teams. Sinister ordered the murder of the reclusive mutants because saw the signs of his work in their mutations. Refusing to let anyone rip him off, he nearly eradicated the Morlocks and left multiple X-Men seriously injured.
Sinister clearly thinks everyone's expendable, including clones of himself, in pursuit of his goals. In one of the dark futures of Powers of X, Sinister's interest in cloning ultimately doomed Krakoa. With a scheme clearly in motion and powerful DNA from Arakko at his disposal, Sinister may be headed down that path.
Of all the former enemies the X-Men have welcomed to Krakoa, Sinister may be the most dangerous. Apocalypse might seem to be the more intimidating threat, but he does care about the future of mutantkind. Sinister only cares about his work. Moira MacTaggart warned Xavier and Magneto against recruiting Sinister, and his inclusion may yet spell Krakoa's doom.