A masterpiece from start to finish.

I don't think that's an accurate assessment.

First of all, Theo (the "hero") decides to help "a bunch of criminals" because he's still in love with their leader (Julian). Money is a motivator as well, but deep down, I think the character was simply clinging to the possibility of rekindling their love. She even blatantly dangles it in front of him to coax Theo into doing what she wants.

Second, that "bunch of criminals" (The Fishes) was a resistance movement, and while they ultimately end up doing some pretty despicable things, their cause is not unjustified. They were fighting an equally malicious government; who's to say they're right or wrong? It's a very grey area. Also, it's not like Theo knew he couldn't trust these people until he finds out that they killed Julian, and by then he's already crossed the point of no return.

Third, aside from the fact that both the love of his life and his only friend are murdered in front of him, Theo risks his life for a total stranger because his eyes have been opened to a greater purpose. What would be the point of returning to London? What exactly is he risking by helping this girl? He's got nothing to lose anymore. The only thing left for him in London is a meaningless life and a lonely death (assuming The Fishes don't find him and kill him first, or worse). He risks his life for a total stranger because he knows that her life means something whereas his doesn't; her life is more important now.

There Will Be Blood is another one that ranks alongside Children Of Men on my all-time top five list, and you're right, they're both criminally undervalued films. Just recently I introduced Children Of Men to some friends of mine who had never even heard so much as a murmur about it. They liked it a lot, and as soon as it was over they starting asking "When did this come out? Why have I never heard of this before?"

I realize that not everyone wants their entertainment to be a deep, meaningful character study, but it's a bummer knowing that so many incredible films never achieve the recognition they deserve in the wake of the multi-million dollar blockbusters (which aren't inherently bad movies, per se, just usually bad movies).

No, same movie, but probably different political views. I have heard people criticize this film for being too liberal, and while I disagree that it's preachy or heavy handed, I'm not going to argue those points here because I absolutely do not want to have a political debate, lol.

As far as lacking sufficient backstory and a satisfactory resolution, I think that's just flat out wrong. Literally everything you need to know about the backstory can be inferred throughout the film, in fact every time I watch it I notice more and more. On the contrary to being lacking, the storytelling here is second to none. It struck a perfect balance between too much and too little information.

Also, I thought the ending wrapped up everything perfectly. Theo didn't die in vain and the girl boards the ship (called "The Tomorrow", btw), leaving us with a tiny glimmer of hope for the human race.