Bob the Goon

The actor who played Bob the goon was already a friend of Nicholson and it's said that Nicholson insisted on him being given the part so that helps with chemistry.

My impression is the script resolved to have the Joker kill Bob at that point to show that the Joker had lost patience with any sense of loyalty, morality and restraint. In film terms, it's also shocking, even though also potentially slapstick comical. The Joker had had his balloons taken away "He stole my balloons" and, like a disappointed child at a party, someone had to pay the Partyman - the person he'd counted on to protect his crazy dream. Bear in mind that the Prince song 'Trust' had just played. Trust was vital to him, particularly after Grissom's betrayal. Something to bear in mind is that the Joker is technically a child at this point because he was created maybe a few days ago. Prior to that, he'd been someone else - Jack Napier. That chemical dunking was like a baptism for a new person, albeit with vestiges of the old one. So, despite what the Joker claimed to Bob, "You are my number one", that's not necessarily as true as it was about Jack Napier's view of Bob. Bob doesn't even seem particularly artistic (was he even at the art gallery?)- no wonder he was expendable to The Artist Formerly Known As Napier.

(ps after that, if you look at him beckoning Batman's plane, it's masterful acting to me. "Come on you son of a.." The Joker sounds like manic joy has gone from his mind - at that moment anyway - and he needs a conclusion. It's quite moving imo when he's desperately pulling out joke glasses and joke teeth fall to the floor. And because he's without Bob, just reliant on heavies, he's in a weaker position. He forced a conclusion on himself by going up the belltower).

I like Bob, but I've always found that to be perhaps the funniest movie in the movie. It's so random and true to the mercurial nature of the Joker.

Originality needs a reboot.

It's foreshadowed that the Joker is going to throw Bob under the bus somehow when he gives him the same phony "#1 guy" pep talk as Grissom.

It's just a recurrence of events earlier in the film... Jack was Carl's #1 guy but he did something that made him unhappy, so he tried to kill him. When The Joker tells Bob "you're my number 1 guy", he's merely mocking the betrayal that happened to him. So it's only a matter of time before Bob gets it.