While it did not create the idea of a villainous Superman, Injustice was the most influential story when it comes to that. As a result, the trope of an evil Superman or Superman equivalent has become overused. That shows how effective the Injustice version is due to his many dark and twisted actions.
Across the Injustice games and prequel comics, Superman's fall from grace was showcased through many evil acts. From the killings of famous characters to destroying the lives of those he once called his friends; the evil High Chancellor Superman has committed many atrocities that render him irredeemable.
Murdered The Joker
Nobody can deny that Joker earned his death for what he did. Batman, Injustice 2, "I understood why. We all did. But once that line was crossed, there was no going back."
Joker used Scarecrow's fear toxin to manipulate Superman into killing a pregnant Lois thus triggering a nuclear bomb that turned Metropolis into a crater. If anybody had a right to kill the Joker out of rage, it was Superman. At the same time, Superman lost his moral com that moment as he sent his fist through Joker's body.
Exposed Batman
Batman remained a constant threat to Superman and his new Regime because of his anonymity. That changed when Superman did the one thing that tore Batman's world down around him: he exposed his secret identity as Bruce Wayne.
This not only exposed Batman but all of the Bat-Family as well as put those he cared about in danger. Wayne Industries froze all of Bruce's funds and assets, leaving him back to square one where he had to make everything himself. Plus, now criminals and villains had a hard time being afraid of Bruce Wayne.
Betrayed Kara
There is a small period where Injustice 2 sets up the potential redemption of Superman. Sadly, he is simply too far gone, despite Kara Zor-El AKA Supergirl keeping their family symbol's meaning alive. She, as well as Batman, try their hardest to make Superman see that the path he's on is wrong.
Instead, Clark made the ultimatum that Kara was either with him or against him. Even Kara points out that at that point, he was less of a Kal-El and more of a General Zod. The only good to come out of this is that Kara was able to take her cousin's place in the Justice League and Elseworld's Finest.
Vaporized Martian Manhunter
Superman's rage got the better of him but more purely of his own accord. Wonder Woman proved to be more of a villain than Superman and the best example is when Martian Manhunter attempted to stop Clark and Diana from killing the Joker.
Wonder Woman could have found any other solution but she pushed Superman to use his heat vision on Martian Manhunter as he was attacking her. As a result, the wise and heroic Martian was burned to ashes because of her and Clark falling from grace.
Took Away Dinah's Husband
Superman knew how much Green Arrow and Black Canary loved each other. Even then, he let his anger take over and he beat Oliver Queen to his death, leaving a pregnant Dinah without Ollie and their son without a father.
Even though Dinah was able to meet another Oliver Queen from a different Earth, she was still heartbroken over what Clark did. As a result, it is so satisfying to see Dinah nearly kill Clark later on in the comics.
Killed His Pregnant Wife
Now, it may not have been his fault but that doesn't take away from the horror of it all. Due to the fear toxin, Superman believes he is fighting Doomsday but it is actually Lois. When the truth is revealed and the reader realizes that he was pummeling Lois at full strength, it's easy for the reader to look away.
It's likely that every bone and organ in her body is already completely decimated but that's not all. Readers are told that Superman hears his own unborn child's heart stop and that's what triggers the nuke.
Murdered Billy Batson
The thing to keep in mind is that Shazam is in reality a kid in a grown man's body. Not just any random child though, Billy Batson is someone that looked up to Superman as his hero. He practically worshipped Superman which is why he blindly followed him in the Regime.
So it makes it extra hard to see Superman freeze Billy's mouth to keep him from saying the magic word and then use heat vision to burn through Billy's head. That was a good-hearted child who simply pointed out Superman's misdeeds and he murdered him for it without any regrets. It's at this point that Superman had become no better than the Joker.
Enslaved The Human Race
To a degree, it's easy to see why Superman wanted to become a leader after what happened to Metropolis. However, he and the Regime went too far, collapsing every government on Earth to create the One-Earth Government with the Regime becoming dictators.
Atlantis was subjugated, the people of Earth lost all freedom, and Superman's Regime had its own military presence that kept every city under total martial law. Protecting people is one thing but Superman started viewing people more like pets or disobedient children which led to Batman battling Superman with the Insurgency.
Arranged Alfred's Murder
Prior to Superman's fall to villainy, he and Batman were shown to be good friends. So even he knew what Alfred Pennyworth meant to Bruce. Alfred was the purest soul which is why the ultimate sin Superman committed was hiring longtime Batman villain Victor Zsasz to go after Alfred for information.
Victor Zsasz did was he does best and murdered the loyal butler after the two fought. Between losing Nightwing, Damian betraying him, being exposed, and most of his friends becoming dictators, Batman had already suffered so much. So seeing him lose Alfred is enough that anyone reading the comics will likely shed a tear or two.
