Bane is VERY much a villain and does not help Batman in the film. Christopher Nolan envisaged and wrote him as villain. His actions throughout are cold blooded and cruel. Despite showing care for Talia, he is merciless in his destruction of all others, murdering at will and threatening to slaughter an entire city.
Much of the theory is centred on the idea that Bane strengthened Batman by leading him on a journey to make him a tougher, more powerful hero and thus was the true 'dark knight'. However, this involves completely misreading many scenes and the countless actions of Bane which prove otherwise.
(Very) Long Answer*
I will try and tackle the key themes of the theory, by posting it from the source in Italics. I will then draw on a variety of source materials to show my disagreement.
The theory begins:
Bane is, in many ways, a bizarro sort of Batman. He is extremely intelligent, he's iron willed, he's physically powerful, and he's got
a justice agenda of his own that got him kicked out of the League of
This statement begins with a correct assertion. Bane is a twisted version of Batman, as Christopher Nolan intended:
The really interesting thing about Bane is, he's massively strong but
he's also extremely intelligent, and his past very much mirrors Bruce
Wayne’s in interesting ways, from his training and with the League of
Shadows background. Bane represents the the wrong path of Bruce Wayne
almost back to “Batman Begins.” So Bane is the return of that danger.
The wrong side for Bruce Wayne.
However, this is where theory begins to go astray. It states his justice agenda led to him being kicked out of the League of Shadows. This simply isn't true. Bane was imprisoned in the Pit, along with Ra's al Ghul's wife and daughter (Talia). The other prisoners killed the wife, but Bane intervened to save Talia and help her escape. She found the League of Shadows and returned with them to free Bane, who was inducted into their ranks. He left because he was a constant reminder of the place where Ra's al Ghul's wife had died - so Ra's had him excommunicated from the group. There was no alternative notion of justice that contributed to this. Throughout the entire film, his philosophy - to destroy Gotham, a city corrupt beyond redemption - is identical to Ra's' goals.
The theory continues:
First, he has to convince Bruce that Gotham needs Batman. Then, he needs to show Bruce that Batman is more than Bruce could ever be.
Finally, he needs to secure a victory for Batman over the League of
Shadows. He gets the League to go all in here. Talia seems to be the
last of the organization, so a victory for Batman doesn't just grant
Gotham a reprieve, it effectively destroys the League of Shadows. If
any of this can't be achieved, the League of Shadows wins, and Gotham
Bane's actions towards the beginning of the film are all designed to install Talia as the head of Wayne Enterprises, so she can gain access to the nuclear fusion core they had. When Batman confronted him and they fought, he crippled Batman and told him what his goals were:
So, as I terrorize Gotham, I will feed it's people hope to poison
their souls. I will let you believe they can survive so that you can
watch them clamoring over each other to "stay in the sun". You could
watch me torture an entire city and then when you truly understand the
depth of your failure, we will fulfill Ra's Al Ghul's destiny. We will
destroy Gotham. And then when it is done and Gotham is ashes, then you
have my permission to die.
There is no intention to save the city, or do anything positive for it - only to destroy it. Now, even if you were to argue (however ludicrously) that this was all a ploy to strengthen Batman, when they had their rematch, and Bane was overpowered, Talia stepped in to save the day and leave Batman in a heap. She left to ensure the thermonuclear bomb detonated, giving Bane strict instructions to ensure he died in the fire. Bane ignored this, as the script shows:
Bane grabs Batman, throws him into the floor - checks the
rounds in the barrels.
BANE - You'll have to imagine the fire -
He shuts the gun - jams it into Batman's face.
We both know I need to kill you
He squeezes the trigger. BLAMMM! Bane is blasted across the
lobby by cannon fire.
Catwoman is there, on the Bat-Pod.
CATWOMAN - The whole no-guns thing? I don't
feel as strongly about it as you
The only reason Batman survived this was because Catwoman happened to arrive at the right time. Again, there is nothing to show he is in anyway helping Batman - just trying to destroy him. The only reason he allowed Batman to survive in the pit was, as shown above, out of a sadistic desire to let him see his city burn - and, interestingly, to echo the treatment he himself had when he was in the pit; a cruel, torturous wait for an inevitable end.
The theory then states:
Batman tries to confront Bane and Bane teaches Bruce a very important
lesson: Bruce isn't immortal or unbreakable. Batman needs to be both
of those things.
Or to put it another way - he beats Batman up! Which is completely canon and sets him up as a truly physical enemy, something that Nolan also intended:
Bane, to me, is something
we haven’t dealt with in the [Batman] films. We wanted to do something
very different in this film. He’s a primarily physical villain, he’s a
classic movie monster in a way — but with a terrific brain.
The theory then postulates:
So then there's the pit, and the quite literal rising.
This is intended and is far more likely a reference to The Lazarus Pits of the comic, something Nolan himself discussed on the DVD. Ra's al Ghul used them in the comics to restore himself - instead, Batman uses them here.
The theory then comments:
Batman and Bane fight. Batman wins this time with some help, and now
it's time to save Gotham. Catwoman is all "you've already given them
everything" and Batman is like "not everything. Not yet." and that is
so fucking key. Bruce had given Gotham his body, his wealth, and his
heart. But he hadn't given them Batman. Bruce Wayne, the angry kid
trying to avenge his parents, didn't come out of the Lazarus pit.
Batman did. All that Bruce has left in the world is Batman, and his
final act in the cowl is to give Batman to Gotham completely. Gotham
will forever have the symbol (and the crime fighter, if John Blake has
Catwoman wanted him to give up because she was ruthless and selfish. This is true throughout the film. She would leave the millions of people to die. Bruce Wayne/Batman cannot do that. So he saves them. However, I'm not sure why that means he's suddenly just Batman. If that interpretation was true, why on earth would he stop being Batman immediately after saving the day. It's not as if there wasn't still crime to deal with.
As for the notion that he has given Gotham the Batman signal forever - he'd already done that. We see evidence of this throughout the film (e.g. the little boy doodling the Batman symbol on the bench). Many people in the city still belived in him.
The theory then finishes:
We're left with a Bruce Wayne who has moved beyond his parents,
Rachel, and Gotham. He had to give up Batman to do that. Gotham has
Batman, is safe from itself, and safe from the League.
This effectively states that because Batman became stronger, Bane is the true dark knight and hero. That's just nonsense. If I were to walk home today and get beaten up by a thug, inspiring me to take fighting lessons which made me stronger and able to defend myself in future, has that original thug saved me or helped me? No! Of course not. They weren't doing good, they weren't a hero. They were a villain. Like Bane. On a much smaller scale.
To throw in a few final arguments, remember that Bane is perfectly content with the plan to detonate a device that will kill millions. He televises this fact and then, in front of the world, murders Dr. Pavel, the only person who could disarm the device. He also goes to the prison and instigates a full riot, all in the name of creating anarchy and allowing Gotham to burn. He murders countless people, setting explosions all across Gotham, including at the football stadium:
There is no logical way to look at Bane's actions as anything other than cold hearted and cruel. He was a villain, as Nolan intended. He slaughtered people willingly and, despite showing care for Talia, showed nothing but cruel hatred for the rest of the world. His actions were evil. He was just evil.
* I've edited this answer quite drastically to make it flow better. The original can be viewed (if desired) in the edit history of this answer.