Colossus has a long history of being a wildcard among the X-Men, even while serving as something of a bedrock for many members of the team. That's what made him such an appealing double agent for enemies of the mutant nation, allowing him to be used as a serious weapon against his nominal allies. But even revealing the truth could do massive damage to the mutants going forward.
Source：https://www.cbr.com/xmen-colossus-traitor-xforce-domino-marvel-comics/
