Instead, we're fed the idea that we've finally found a horrible thing that Julia isn't comfortable doing, although up until then she has shown remarkably little concern for anyone who isn't Julia. She doesn't sweat running that guy over. She starts treating Tom with more compassion (although it seems like that is only because she thinks it serves her, she didn't give a second thought to leaving him in the fucking desert). And then, as you point out, she gets Diego killed.

They never laid the groundwork for her to do anything BUT look out for herself, and then they want us to believe that she is willing to put herself in harm's way just to protect Tom. I don't buy it.

I don't know that getting the audience to root for Julia is that much of an accomplishment. It's achieved primarily by contrasting her with more hardened villains who are even less savory than herself. I definitely think part of the reason you start to root for her is because you've watched her get in way over her head and then start to adapt, and that's worth something. But otherwise, you just feel like this shitty woman should get the money instead of the shitty other kidnappers. And we might all feel differently if we'd spent the first 90 minutes with the those Mexican kidnappers.

Simply making her more tolerable (she's super annoying early on), makes the rest of the movie more enjoyable, and I'm thankful for that. But I don't believe that she has changed at all. Although the ending left me thinking that the filmmakers wanted you to believe she had.