why did Ben Wade kill Charlie?

I had no problem believing this scene.

Ben Wade said he was alone from young after his dad died and his mum abandoned him at a train station with a Bible. He probably fell in with bad apples from very early on after that who influenced his whole outlook on life from that point on.

After that, how often do you think Ben got the chance to witness the power of good and being a moral person? I doubt he was the type to sit around for a sermon or lecture on being good and just.

However, with witnessing Dan's goodness, he started to have an epiphany. You could see his change when drawing Dan sitting watch in the window. I think he realised it's not man's place to take what he wants, and people like Dan keep the world going.

There's also another aspect to this... Ben was genuinely touched by Dan's interaction with his son at the end, something Ben probably has no recollection of receiving in his own childhood. I think then he started to realise he and his gang were just rejections from society and the real reason they were evil.

Then the final straw came when Dan, the only example of good powerful enough to have touched Ben in his entire life, gets extinguished via a shot in the back by Charlie... hence Ben's revulsion when he performs a coup de grace on him.

Yes, exactly, Charlie was loyal and Ben was not, so why does the movie celebrate Ben? Charlie was the real hero. Even when they were all in the gang, Charlie was the MVP, they should have just left Ben like the other guy said, but loyal Charlie insisted they go get him. Russell Crowe's character sucked for a lot of reasons, this was Foster's movie, he should have shot Ben after shooting Dan, who sucked just as much.

I truly hated this, would have been perfectly okay with me if he rode off with his gang!