The question from me is: Did the titans within the walls become the wall willingly. We've got Eren-titan fighting for mankind, there's no reason others can't do the same.

It's entirely possible that while titans were made as either:

1) an accident
2) a way to advance the human race
3) a way to unite humans with a common enemy
4) a way to cripple the human race,

that some titans thought the last of mankind should be preserved, and used themselves to become the walls of humanity.

Perhaps it's Annie's goal to liberate these unsung heroes as the current humanity is a piece of degraded shit anyway? The titan in the wall had that peeled look, and the only 2 peeled titans we've seen are intelligent, and to all intents and purposes - human. I really do sympathise with Annie even though we know close to nothing about her still, mainly because she feels remorse about all the lives that have been sacrificed. Whatever her goals are, she sees that it's worth it in the end. Eren just rampaged like no other in his own sense of justice. I'm not going to argue that the justice is misplaced, but he's not looking around to see where it's taking him. Perhaps it's his lack of control over his titan form. Or perhaps he's just simple-minded.