His brother was dead.
If Mycroft had mentioned this fact to someone else, such a person would likely have looked at him askance. A braver soul might even have said something like: “Of course he's dead. He jumped off the roof of a building over a year ago.”
It wasn't true, of course. His brother had left the building alive and well, the plan to fake his own suicide executed perfectly. He'd then run off to travel the world, taking down the remains of Moriarty's web all by himself. Mycroft had given him all the help he could without letting anyone in on the secret.
It hadn't been enough.
When Sherlock had taken down the first of Moriarty's lieutenants, the other two had been tipped off immediately. Sherlock was dead within the day.
It had taken Mycroft a week to learn of his death; it had taken him only two minutes to realize he would never accept it. Nothing else in the world mattered anymore. He didn't care what he had to do or how he had to do it, he would get his brother back. If it meant altering reality itself, then so be it.
With the resources he had available to him, he found a possible means of accomplishing his goal almost immediately: a time travel project created by the American military. It had been abandoned when the genius scientist in charge of the project had vanished into thin air along with the prototype.
Fortunately, he'd left some perfectly good schematics behind. Mycroft only needed to work out how to use them.
Mycroft threw himself into his new project with a dedication he'd never known before. He was intelligent enough that he'd never had to truly work hard at anything. Certainly, most people who met him got the impression of a diligent, hard-worker, but he was exactly as lazy as Sherlock had always claimed.
Sherlock would be shocked to see him now.
Within a month of his brother's death, Mycroft not only had a full understanding of the device, he'd made a few small improvements of his own. The original device worked, of course – in fact, Mycroft was certain that the scientist who'd invented it was off enjoying a branch universe out there somewhere – but it only allowed the user to travel back in time. With the modifications Mycroft had made, he'd be able to travel forward along the branch in time, to see the ultimate results of his interference.
He knew this would likely result in the creation of several different branch universes just to accomplish this one goal, but that didn't especially bother him.
Once he had the machine, he ran into a peculiar dilemma. Namely, figuring out the best way to use it.
Mycroft's initial thought had simply been to go back to just before Sherlock's death and save his life, but he'd written off that plan immediately as being too simplistic. Sherlock's life would still be in danger, and what of the next attempt?
His next thought was of going back to before Moriarty had allowed himself to become known to Sherlock. Mycroft could easily have him killed.
However... he'd had his reasons for not doing it that way the first time around. Moriarty's network would still be a huge threat.
Mycroft realized then that what he really needed to do was get rid of Moriarty before he even had a web to threaten Sherlock with. He went back through the files he had on Moriarty, looking for how far back he'd have to go to prevent any of it from coming into existence in the first place.
Unfortunately, it seemed that the optimal time was Moriarty's ninth birthday, just before he'd killed Carl Powers.
Whatever Moriarty might have done, Mycroft still found himself reluctant to kill a child. And given that he himself had been twenty at that time and not yet in any position to order a hit, he'd pretty much have to do it himself if he went that route.
And so it was that Mycroft ended up choosing a different approach. Because, when he got right down to it, he didn't really need to kill Jim Moriarty. He only needed to kill any chance of Jim Moriarty growing into the sociopathic criminal mastermind he'd become originally.
And there were plenty of ways he could manage that.