Warning: Spoilers for Batman Fortress #8 ahead!For all of his incredible power, Superman is well known to be bound by an unbreakable code: he does not kill, and he does not allow others to kill. No matter what, Superman always seeks to offer his foes a chance at redemption, and barring that, justice according to the laws of their people. Therefore, when he's shown to stand by and do nothing when Lex Luthor is murdered in Batman: Fortress, it is the ultimate violation of his moral code.
Throughout the events of the series, an alien invasion has left much of the world without power and Superman is nowhere to be found. In a bid to stop the crisis, Batman gathers a team consisting of Lex Luthor, Emiko Queen, Green Lantern Day'l and Jackson Hyde to try and break into the Fortress of Solitude to locate Superman, unaware that the invaders are also searching for him. This all culminates in a showdown between the three parties - Batman's team, the invaders, and Superman - in the heart of the Fortress.
Superman Isn't Bothered When Lex Luthor is Brutally Killed.
Gary Whitta, Darick Robertson and Diego Rodriguez's Batman: Fortress #8 shows the bizarre conclusion to this conflict. Learning that the invaders have sought out Superman to have him stand justice for the House of El's ancient crimes, Lex quickly tries to sell out Superman. Disgusted by this act of betrayal, one of the invaders nonchalantly murders Luthor while Superman stands idly by, doing nothing as his nemesis is crushed into a bloody cube. It is only four pages later, after Batman and his team have started to fight the invaders, that Superman finally calls for everyone to stop. Even then, Luthor's death is not even acknowledged.
This depiction of Superman goes against everything he stands for, and is made into utter hypocrisy when he later submits to the invader's judgment, saying "all my life, I've said I stand for truth and justice. These people deserve those things too." This flies in the face of his passive acceptance of Luthor's demise, where the man is brutally killed with no due process or accountability for his crimes. Case in point, when Superman's former ally Kryl-Ux slays the tyrant and warlord Mongul in Phillip Kennedy Johnson, Brandon Peterson, Will Conrad, Max Raynor, Miguel Mendonça and Lee Loughridge's Superman: Warworld Apocalypse, Superman can only regard Kryl-Ux with cold disgust as the murderer makes his escape. Superman will not permit murder when he can prevent it, full stop.
Superman Only Steps In Once His Friends Started Fighting.
Superman is well aware that with his immense power, he cannot ethically stand as judge, jury, and executioner over the world. Modern depictions of the character show him taking great pains to not cause undue harm and to extend kindness even to his enemies. Having him as a mute bystander as Lex Luthor is brutally killed before his eyes isn't just bizarrely out of character for Superman; it also violates every part of his moral code.
Batman: Fortress #8 is now available from DC Comics.