How did Colossus die?
Colossus died as a result of the Legacy Virus. Beast had come up with a cure, but it required a host for it to be spread from, just as the virus did; the catch was that the host would die, so they decided to shelve it while Beast found a better cure. After he lost his sister to the Legacy Virus, he decided he didn’t want that happening to any other mutant, so he injected his shit and died.
