Black Mask was a pale imitation of his ultra violent and bi-polar comic book version at least the first Black Mask which the film version uses.
There was enough violence in this movie as it was. At least most of it served a purpose. Having this guy keep a bunch of people in his office just so he could beat them up when he was mad was just stupid. For one thing, he'd be constantly breaking his fists. For another, there's no real menace in a character that full of uncontrolled, childish rage. He just seems pathetic. I mean, the Joker randomly killing people is effective here because it's not out of anger; it's done as a joke, or to make a point (which can be the same thing). It's scary because the Joker is perfectly calm and in control. That gives him psychological power over the scene, over the other characters. The Black Mask was so panicked and out of control that he came off as powerless, a weakling using yelling and hitting to compensate for his lack of psychological power. And so he wasn't at all scary. His scenes were just tedious.
Black Mask was a means to an end anyway. Jason needed to get the Joker's attention and used Black Mask by attacking his organization and making him angry enough to hire the Joker to try and off him to do it.
Yes, I did see the movie, thank you, so I know that. That doesn't mean the character's portrayal had to be boring and gratuitous.
As for the writing...since Judd adapted this from his own story arc almost all of the dialogue was lifted or reworked to fit the film. Amazo is from the arc as well.
Exactly why slavish adaptation isn't always a good idea. The kind of random crossover that can work in a medium like comics, where it's all interconnected, is less effective in what's supposed to be a standalone movie. Indeed, in a lot of ways, this story wasn't ideal as a self-contained work. I'm not sure what a viewer who had no prior familiarity with Jason Todd's story would make of it. For one thing, the whole Ra's al Ghul interlude seemed an awkward interpolation that dragged the story to a halt for the sake of labored exposition. I know that Ra's was a stand-in for Talia in the original story, but still, it might've made for tighter storytelling if the explanation for Jason's resurrection had been simpler -- if, say, there hadn't been a body and he'd never actually died in the first place (though I grant that it would've been hard to convince Batman of his death that way).
And I know that Winick wrote the original, so I took it as a given that much of the dialogue was from the comic. That doesn't change the fact that it isn't very good dialogue. I would've been happier if someone else, someone with more of a gift for dialogue, had written the script. Naturally Paul Dini is the first name that comes to mind for a Joker-heavy story.
I thought it evident why he was used, Black Mask wanted him to kill Batman thinking an android who could absorb superpowers would be able to defeat him.
Maybe in the comic, but definitely not in the movie. Black Mask wasn't even expecting Batman to intercept the shipment. He said outright that he intended to sell Amazo to foreign powers and thus expand into international arms dealing.
The villain was also obviously used to showcase how well Bruce and Dick work together and I think it was executed well enough to get that point across.
But that could've been done with any powerful character, and surely there are candidates in Batman's wheelhouse, such as Clayface and Bane. (Besides, "Amazo" is just such a damn stupid name. And the character design makes it even stupider. I've only ever seen the JL/U interpretation, which was radically different.)