Why did Joker kill Monster T?

Because joker is crazy and mr t was checking out harley

he called Harley a bitch.

imo

Its the Joker, the guy who shoots his own men for giggles.

I think they realized they needed to show the Joker being CRAAZY early on, because other than him waving the machine gun around later he doesn't really kill anyone else in the movie.

Monster T's death is weirdly (awfully) edited, making me wonder if it was "added" in post. There's a quick shot of Joker's henchman staring as Monster T says "No, J!" offscreen, then a cheap white flash clearly added in post, then a weird shot of his head falling, then a weird head-on shot of Joker holding the gun...

For looking at Harley and calling her a "bad bitch". He's very possessive of her