Spoilers ahead for Episode 18 of Gotham Season 4, called "That's Entertainment."
Let it not be said that Gotham is unafraid to kill off major characters! The latest episode was a bloodbath that saw a lot of murder and mayhem courtesy of none other than Jerome Valeska, and he was on the verge of driving masses insane courtesy of the gas he commissioned from the Scarecrow. After getting his hands on major figures like the mayor, his brother Jeremiah, and Bruce Wayne, Jerome seemed poised to win the day. Gordon and the GCPD did get the upper hand, and Jim shot Jerome a few times before Jerome let himself slip off of a rooftop and fall to his death. Jerome seems really, truly, genuinely, 100% SUPER dead this time around. R.I.P. Jerome. As much of a shame as it is to lose Jerome, his death means one thing: the Joker's time has come, and it's none other than Jeremiah.
Before his death, Jerome managed to play one more mind game on his brother when he told Jeremiah that they're both totally crazy, and Jeremiah is just better at hiding it. Jeremiah was visibly bothered by Jerome's words, which was a sign that there's something to Jerome's claim that something is fundamentally off with both of them and Jeremiah knew it. Jerome, like the expert madman that he was, didn't let death stop him from knocking Jerome off his rocker.
At some point before his death, Jerome sent his brother a package that seemed to come from Wayne Enterprises. Jeremiah opened it and was sprayed in the face by a variation of the fear gas that Jerome had meant for the folks of Gotham City. He wanted to unleash Jeremiah's madness, and based on the final scene of the episode, it's safe to say that Jeremiah is indeed the Joker we've all been waiting for.
The gas contorted his face, put a decidedly creepy and Joker-esque grin on his face, and gave Cameron Monaghan the chance to go full-on Joker. Gotham delivered on two of its promises: that the Joker would turn up on the show soon and Jerome was not the real Joker. As it turns out, he's both figuratively and literally the proto-Joker. His madness did inspire all the crazies to come out in Gotham City and certainly pushed Jeremiah close to the edge. Then, he literally created the Joker by exposing his brother to the gas that physically transformed his brother into somebody even more dangerous than Jerome himself ever was.
Why could Jeremiah be more dangerous than Jerome ever was? Well, it all has to do with Jeremiah's life before he was knocked off his rocker. Jeremiah was an academic with a higher education, and his proposals for Gotham impressed Bruce so much that he insisted on giving Jeremiah a grant from Wayne Enterprises. His academic background combined with the tactical smarts shown by Jerome even at his craziest that he probably shares means that he could be more of a risk to innocent lives than Jerome ever was. Jeremiah the Joker is much more formidable than Jerome the proto-Joker.
Jeremiah also has a reason to target Bruce on top of any blame he may level at Bruce for what happened to Jerome. The package that contained the transformative gas came in a box that seemed to be sent from Wayne Enterprises, which meant that Jeremiah had no reason to suspect that something was up with it. Obviously Bruce had nothing to do with what happened to turn Jeremiah into Jerome, but the newly crazy Jeremiah might not see things quite so clearly.
He may also find himself with more fond feelings for Jerome now that he's become the Joker, and feeling fondly for his brother means hating Bruce Wayne. All things considered, Gotham used the death of Jerome to introduce the Joker in a truly epic way. Jerome needed to die for the Joker to rise up, and I'm psyched to see what happens next. Please, Fox, do not cancel Gotham. Season 5 could be the best yet.