Colossus has always been one of the strongest mutants, but one X-Men conflict led him to kill a foe that even the Hulk couldn’t have stood against. The reality-warping Proteus is one of the most dangerous mutants in existence, and only through his specific power set could Colossus do what he believed had to be done to stop Proteus.
Piotr Rasputin aka Colossus has the ability to turn his skin into "organic steel," granting him superstrength and resilience. This power set made Colossus the main muscle of the second generation of X-Men, the team introduced in 1975's Giant-Size X-Men #1. Throughout his history, Colossus has always been one of the Marvel Universe’s physical powerhouses, tangling with foes from the Juggernaut to the Hulk. However, it’s not his raw strength but his metal skin that gives Colossus an edge over one of the most powerful villains that the X-Men have ever faced.
In Uncanny X-Men Vol. 1 #128 by Chris Claremont and John Byrne, Colossus is able to beat Proteus, a mutant whose body-hopping Omega-level psionic abilities have made him nigh-unstoppable. Proteus, or Kevin McTaggart, is the son of Moira McTaggart, who kept him hidden away on Muir Island because of his power and mental instability. When he finally escapes, the X-Men realize that Colossus is immune to Proteus’ power while Piotr is in his metal form, and Colossus makes the decision to kill Kevin to prevent him from endangering anyone else. Colossus’ victory is doubly impressive because Kevin is an Omega-level mutant, the term for a mutant with no upper limits to their power. In Exiles Vol. 1 #75-77 by Tony Bedard and Jim Calafiore, an alternate Kevin is even powerful enough to even take down Marvel 2099’s Hulk.
Colossus' Fall From Grace
Often characterized as a gentle giant, Colossus’ killing of Proteus paves the way for a darker version of Piotr Rasputin, and has shaped his characterization for decades since. When Mr. Sinister’s Marauders begin killing the subterranean Morlocks in the Mutant Massacre, Colossus kills their member Riptide in Uncanny X-Men #211 by Claremont, John Romita Jr. and Bret Blevins. This darker turn would continue as he loses his sister Illyana to the Legacy Virus, culminating in him joining Magneto’s Acolytes after his disillusionment with the X-Men. Without him killing Kevin, it's arguable that this chain of events would never have happened, and Colossus as fans know him today would be a vastly different character. This darker side of him is still present to this day.
In the current Krakoan era of the X-Men, the ties between Colossus and Proteus have the potential to change the future of mutantkind forever. A heroic Proteus is currently one of the Five, the mutant circuit that allows for mutants to be resurrected. This role makes the Five the most important mutants alive, and their survival is key to Krakoa’s. Meanwhile, Colossus is sitting on the Krakoan Quiet Council, privy to some of the biggest secrets in mutantkind, but he isn’t currently himself. Instead, Colossus is currently under the mental manipulation of his brother Mikhail via the mutant known as Chronicler. If Mikhail decides that mutant resurrection needs to suddenly disappear, then Colossus’ unique ability to take out Proteus —a foe even the Hulk can't defeat — could suddenly become a lot more of a problem for not just Kevin or the X-Men, but for all of Krakoa.