WARNING: The following contains spoilers of Injustice: Year Zero #14 by Tom Taylor, Roge Antonio, Rain Beredo, and Wes Abbott, on sale now.
Injustice: Year Zero has finally come to an end, and it's a brutal one at that as the Lord of Chaos Joker tries to obliterate the Justice League. After blowing up the Hall of Justice and seemingly disintegrating the Justice Society of America, the Clown Prince of Crime has upgraded his arsenal by mind-controlling Superman. However, Batman still has a few tricks left up his sleeve as he tries to turn the tide.
While Injustice:Year Zero #14 does have the heroes winning the day, it still concludes with Joker gaining sinister clarity into the Man of Steel's major weakness, inspiring him to go on and break the Kryptonian in the video game story.
With the heroes battered and bruised, the Joker turns his gaze to the remnants of the JSA, ordering Superman to kill Amazing Man. However, the Dark Knight appeals to Kal-El's humanity, reminding everyone he's "the best of us." It infuriates Joker as Superman struggles to take orders, with even his chaos amulet telling the clown that Kal-El is incorruptible. This opening allows the Kryptonian to resist and use his super-speed to seize and shatter the amulet, robbing Joker of his power and leaving him open to some deserved blows.
With the war over, Black Canary puts Joker in the hospital by bashing his face in, exacting revenge for what he did to her mom and the JSA. But this is where the pieces fall into play regarding Joker's vendetta to come. Harley visits him and while the clown admits he's found something to live for, it's not her. He reveals he looked into Superman's mind and realized he was Clark Kent. As a result, he's discerned the truth about Lois, which means he knows she's been undergoing pregnancy treatment. This foreshadows his next sadistic move which would kickstart the Injustice war where he'd use Kryptonite and Scarecrow's fear gas to trick Superman into murdering Lois and their unborn baby.
As for the why, it's partly due to his failure to make Kal-El his weapon as Joker was counting on Superman being his apocalyptic toy but mostly, it's due to the smug Bat. He hates how Bruce put Kal-El up on that pedestal, smirking and gloating that Joker could never find a way to break the alien. Joker already went after Batman's idols, knowing that by taking out the JSA it'd hurt the Caped Crusader, and now he's fixated on the person the Dark Knight admires most.
It's not just about destroying the symbol of heroism; it's about graciously accepting the challenge and making Bruce eat his words. The gauntlet's been thrown down as Joker now moves on to prove that Superman is actually the worst of them all, which we'd eventually see when he became a vengeful dictator upon the loss of his family.