There is no denying that Superman, during his Injustice tyranny, turned into pure evil, but it's hard not to at least understand where he is coming from when readers watched the murder of Lois Lane and Superman's tragic villain origin story firsthand. What becomes harder to understand is how he's hoarded a devoted allegiance of followers and comrades while still being clearly out of his mind. Particularly when one of his most devoted followers is the typically sensible Barry Allen, a.k.a. The Flash.
Chapter 27: Barry, featured in Injustice: Year Five #14 by Brian Buccellato and Tom Derenick, opens with a flashback of The Flash talking to Iris five years earlier, shortly after Superman first killed the Joker following Lois Lane's murder. They both recognize Superman went too far in the moment, but Flash believes that he still needs to "be there for him and help keep him in check." He promises Iris that he'll stand up to him if Superman ever goes down the wrong path and will never actually condone his actions. Fast forward five years later, and Barry has watched Superman go too far for half of a decade now. He actually has the opportunity to fix everything by going back in time to stop the Man of Steel, per Batman's request, but Barry Allen refuses. His excuse? He thinks this tyrannical world is what the citizens deserve and they should just accept it.
In five years, The Flash has failed to keep Superman in check like he promised he would. He's neglected to hold the Man of Steel accountable for his many questionable, villainous actions. The Flash may not be the same level of evil that Superman represents, but being compliant in the face of villainy is a form of evil in itself.
It's not like Barry doesn't recognize what Superman has turned the world into is wrong. Chapter 27 also featured an argument between Flash and Superman, after which the latter watched the former save Batman's life, knowing Batman and Superman are in the middle of a war with each other. That argument nearly went physical when Supes almost took Flash's head off, had the Fastest Man Alive not dodged it. In the middle of it all, Flash openly criticizes Superman for acting like an emperor with a growing body count. Flash makes it clear here he knows that Superman has become a problem, yet, despite his own self-awareness, continues to be a part of his team, and thus, part of the problem.
There's no denying that Clark Kent has fallen far off the deep end in Injustice, but at least readers can trace what went wrong with Superman. He'll always have the excuse that he inadvertently caused the deaths of his wife and child, which, to Superman's credit, isn't the worst excuse in the world to turn to villainy. The Flash fails to have such an excuse. The Flash's worst crime is his compliance against Superman and that may be even worse.