Jeremiah didn't go crazy because of Jerome he was already crazy. He did say that Jerome's insanity gas fail and only had mild cosmetic effects on him. Which explains why Jeremiah has the pale face of course. But who knows Jerome and Jeremiah could have worked together on using the gas because it seems that Jeremiah is using the gas as an excuse to start causing chaos. It seems everyone believes he just came insane because of Jerome. Like an example Ecco said " It's the gift Jeremiah's brother Jerome gave him." Jeremiah could have been insane his whole life or after he left the circus.