She can be rational, when the situation calls for it. That's not the real problem with Lieberman's run, I think.
Within the storyline, she's a bit cash and time strapped, and is working with what's availiable, whatever she can get her hands on, quickly, like guns and traditional weapons. Getting those discs was supposed to be an easy, high paying, job that was going to set her up nicely, for a while, but things spun out of control. At this point, it's just after this bit earlier in the issue...
...and she's trying to track down whoever got her into this mess. She suspects the Joker (understandably, considering their history), and she does get very hardcore in these situations. The Joker's response to her attempt on his life is actually in character, too (if one thinks he has feelings, however twisted, for her. I know not everyone agrees with that). He wants to get back together, her trying to kill him being something he finds attractive. Everyone talks about Harley's addiction to him, but very few talk about how screwed up the Joker is in his attraction to her. At least she was delusional about him and their relationship. What's his excuse? Lieberman even wrote one of their best exchanges, where he responds to her saying "I hate you!" with "What's hate but love with some friction?"
The problem, I think, was the tone of Lieberman's run. Lieberman admitted that the shift in tone from Kesel's run was too abrupt, that he should have done a gradual shift before starting Vengeance Unlimited. Taken on it's own, I think Vengeance Unlimited is a good story, but... I think this shows what happens if you go too dark with Harley. She's primarily a light, fun, character. Lieberman put her in this dark, more realistic, setting, where she's still doing her quips (Some of her better ones, even, like responding to what seems to be a SWAT team ordering her to "GET ON YOUR KNEES!" with "Maybe when we get to know each other better."), but the tone just doesn't suit her.
I'm curious to see how Dini's going to handle the balance. He handled it well during Harley's Detective appearances, but how will it work, long term?