Wasn't sure to ask this here or the X-Men board, but since this pertains more to the universe as a whole, I thought here would be better.
The timeline of Old Man Logan happened when Spider-Man rogue Mysterio used his illusions to trick Wolverine into thinking that supervillains were attacking the X-Mansion, and he killed all of them to prevent them from harming his teammates and the mutant children. Except, Wolverine actually killed all the X-Men, and Mysterio fooled him into doing it with an enhanced gas. It was on the behalf of his supervillain alliance, kickstarting the plot of that universe.
Despite normally being presented as a joke at worst and a minor villain at best, this was his most infamous feat in the comics entirety and something Mysterio fans can write home about in regards to his danger and evil potential. In fact, this act of pure evil might've been an inspiration for how FFH's scumbag characterization of Mysterio was based on.
However, it should be noted that OML is an out of continuity story. Say that Mysterio pulls this off in the main universe, under similar circumstances in an alliance with other villains. Thus, main status quo, and the Avengers, Fantastic Four and the other superhero community, the rest of the mutant community, as well as the world at large, reacts to it. How do you think it all goes down?
Thought it would be interesting.