The film fizzled out with a semi-surprise ending that was not really much of a surprise if you stayed awake and paid attention, but that's not its main problem. I can usually live with a disappointing ending if the rest of the movie was good.
The problem with .45 is that it features a whole lot of women with agency and brains, yet for some reason requires the old and tired "woman uses womanly viles to get what she wants" formula, and worse, frames that as empowerment.
Milla plays a battered woman who is too scared to leave her abusive boyfriend Big Al, and too weak to break the unhealthy dependence that feeds the broken relationship. When the abuse has her fearing for her life, she resorts to having lots and lots of hot sex (after all, it's Milla) with people she tries to manipulate into killing Big Al -- all behind his back, knowing full well that his jealousy may get her battered again, or possibly killed.
And that somehow is "empowerment" -- not offing him herself, not running off alone, not even the most boring option of pressing battery charges and putting him away for that. No, it's using "lips, hips, tits, all woman" that gets her "strong" and puts her "in charge."
Quite disappointing indeed.