Why did Jack Hardemeyer hate the Ghostbusters so much?

I dont either. Perhaps he was friends with Peck or got drenched in marshmallow. Think he was just a hateful person full stop

The Ignore Button. Kills trolls dead. ;)

Probably jealous of their success. For a very brief period they were heroes and he was still stuck being some pencil pusher behind a desk. He used whatever piddly amount of power he had to screw them over so he'd feel better about himself.

I'd imagine that a lot of folks in the area dislike the boys in grey after the Marshmallow Man incident was successfully blamed on them.

Wouldn't be unreasonable to suggest that Lenny's approval rating may have been impacted by scapegoating the same guys he brought on-board to handle the issue, which would give someone with the position of his assistant extra motivation to dislike him.

Plus the way he was smiling when he'd seen the Ghostbusters get sent to an insane asylum shows what a total jerk he really is.

Not sure he actually hated them, but if he did, it might have been because the Mayors affiliation with them nearly cost him reelection.

I never understood why the whole city turned on the Ghostbusters, especially after the entire city had seen ghost, or atleast the Marshmallow Man. But none the less they became "persona non grata" after that, and it was people like Hardemeyer's job to not only keep them away from the Mayor, but to try to make people forget that the Mayor had hired them. A big part of doing this was keeping them out of the public eye.