Near the end of Batman (1989),
the Joker kills his trusted henchman Bob with Bob's own weapon.
This occurs right after Batman has done away with Joker's poison-filled balloons.
Why? It seems like Bob was fairly capable. I guess it's probably because Joker was crazy.
According to the film's official novelisation, it was out of frustration that none of his henchmen (including Bob) had told him about the Batwing.
“He stole my balloons!” he screamed.
Nobody had an answer for that. He shook his fists up at the sky.
“Why didn’t somebody tell me he had one of those things?”
Still, nobody spoke. The Joker found this immensely unsatisfying.
He asked for Bob’s gun and shot him. Good old Bob.
That made him feel a little better.
There's also the implication that it was pour encourager les autres.
The boys ran away.
“It’s just cookin’ good, you schmoes!” the Joker yelled at his retreating troops. “What’s going on?”
The boys didn’t even bother looking back. What kind of loyalty was that? And after he had made such a good example of Bob. The Joker hated to waste a good example.
I too remember being shocked by the Joker shooting Bob the Goon near the climax of Batman. The reason he does it is apparently to show that the Joker is in the process of completely losing it. Up to that point, he has been clearly insane and pointlessly, murderously violent. However, he takes some care that he does not easily get caught while executing these schemes. (For example, the way he pollutes the chemicals in various cosmetics to make them poisonous is too complicated for the police to figure out, although Batman, knowing of the Joker's connection to Axis Chemicals, does figure out what is going on.)
Killing Bob shows that the Joker is completely losing control. His homicidal madness is becoming so wild that he just lashes out in rage at the nearest target when Batman breaks up his most recent plan (using the Batwing to take out the poison gas balloons). The Joker is no longer capable of making decision according to even a modicum of self interest. He has become a totally animalistic monster.
His casual murder of Bob shows the audience that the reasonably careful Napier is completely gone, and the crazy, uncontrollable Joker is all that is left. It also serves another plot purpose, since the betrayal and killing of Bob ensures that the Joker has no goons left, so that the final confrontation with Batman in the cathedral can be a one-on-one face-off.
Bob being murdered generates sympathy in the audience for him, suggesting this was leveraged specifically to generate antipathy for the Joker in the final act so the roles of the hero and antagonist become clearer.
Anyone still rooting for the Joker has to reconcile that he has no loyalty to anyone, which is mirrors the allusions to Bruce Wayne's vigilantism is purely self-centred and ignoble - however killing Bob demonstrates the line where this traverses into psychopathy, helping to show the audience where to place their faith.
ie: if you haven't cared about:
Bruce's parents being murdered: this is confusing for the audience because without this event there is no hero in the narrative. Sympathy is indirect.
Gotham's citizens being at risk: this is portrayed comically - it's kind of amusing, not exactly visceral.
You'll still probably want to see Bob's death be avenged, he's a character we've become attached to and we witness him being murdered indiscriminately.
I think it is less that Joker is "losing control" and more just a revelation about who and what the Joker really is. He NEVER gave a damn about Bob. Joker is 100% selfish to the core. He is incapable of concerning himself with anyone else's well being. Bob was the most loyal right-hand man possible. He defends the Joker with his life, he follows every order to a tee without hesitation, he even laughs at the Joker's attempts at humor. There is literally no way Bob could serve the Joker any better and this means absolutely NOTHING to the Joker. Bob has earned no favor. The Joker favors nothing but his own desires. Loyalty means nothing. To the Joker Bob is a tool. This wasn't done in rage that the Joker will regret. Bob will never even cross the Joker's mind again.
