Greetings. I posted this same theory at the end of another thread but considered rewriting it and posting it as it's very own thread after I discovered crazy theories DO have a place here (viewtopic.php?f=5&t=182
I've just watched the Brothers Bloom for the 4th time and doing so allowed me too not just think about the story, but the nitty gritty details. I noticed this time through a couple of details that seem to maybe have some correlation. It might sound crazy and heartbreaking but please hear me out.
One of the first questions I pondered this go through was how Diamond Dog knew what the brothers were up to, how he knew what train they were on when they went to Prague, or how he found Bloom in the hotel room when Bloom was drawing Penelope. The first times I watched this movie I just figured it was unimportant to the plot. But this time I considered what if BANGBANG tipped off DD on the brothers whereabouts.
This lead to the thought process of, is that really possible? The more I thought about it the more it seemed it could be.
-Bang Bang was mysterious. She had a cell phone no one really knew about, she appeared out of nowhere in the brothers' lives, ready to help them with their cons (so she obviously was in the know of the con artist world... it wouldn't be surprising if she also knew DD as well)
-After Stephen was taken, and Bloom was loading the gun in the green VW bug, BangBang appears in what Bloom first believes to be the Russians' car. How curious.
Therefore, what I'm theorizing is that DD, the supposed illusive con master, placed Bang Bang with the Brothers after their incident together because DD wanted to stay close, but being hated, couldn't. Bang Bang was all part of a long con thought up by DD for a very long time, for revenge I suppose. This long con included DDs message to Bloom, that one day Stephen would fall. Bang Bang was essentially the eyes of DDs con by acting as "the muscle" in the brothers' cons.
But it was obvious that Bang Bang had developed sort of a relationship with the Brothers. So why would she con them, right? This still fits in with the theory, however, because while she still remained loyal to DD by continuing to carry out the con, she also got emotional when the actual murdering of Stephen was about to take place. She looked at Bloom at the hotel very sadly, at first I thought it was because they intended to make Penelope believe them to be dead, but in this theory it could mean she knew what was about to happen to Stephen and then what would happen to Bloom because of it. Then, in Bang Bang's final scene, an explanation for that being her "time to leave" could be that she didn't want to stay around and see what she was partly responsible for. She carried out the con, for whatever reason, for DD, but didn't want to actually experience it.