Theory The reason behind it all...

[Brother Coa]

For all of us who read the manga until the last chapter this is the question that bothers us a lot. Because in chapter 1 we were given simple explanation and simple plot but over the time many things occurred that were rather strange to totally unrealistic.

Those include:
-People being able to shift into Titans.
-Those same people having a secret agenda that involves killing every living human inside walls.
-Certain number of people supporting titan shifters from inside the walls, most notable the church and the royalty.
-Walls being made out of living 50m class Titans.

In addition to that we have also learn these:

-When humans run away from titan original onslaught the walls were already constructed.
-Eren's father was very strange scientist who seems to know a lot more about titans then meets the eye, also responsible for making Eren a Titan shifter.
-Titan Shifters are all on a mission to become 'warriors', that mission include killing all the humans inside the walls and keeping that a secret along the way.
-Ymir seems to failed that mission before and now it must return Christ to redeem herself.
-Priest from Wall Religion seems to know entire thing but it seems that thing is so big that he cannot tell anyone if he plan to live on.
-From miner's story we learn that walls also go deep beneath the ground and not just around it.
-The Ape Titan being very interested in what humans have made after all this time behind the walls.

So this is my theory when I see all of this:

There is another kind of human society, made only from titan shifters. They consider itself superior while they consider ordinary humans beneath themselves. In their society young shifters must prove themselves if they want to advance or become something else ( like Annie, Bert and Rainer with their 'I want to become a warrior' part ). To prove themselves they had to do following: infiltrate ordinary humans that they keep in control behind the walls and destroy all of them in certain amount of time. After they have done that they have proven themselves as warriors and came come back and receive their new status in society.

The main factors backing this up:
-The entire setting of walls, it all seems set up as a perfect cage for test animals or for controllable group of animals.
-The fact that titan shifters kill tons of people while living every day with them.
-The fact they are caring for others titan shifters but not for rest of humanity itself ( except Ymir who fell in love with Christa ). Even going as far as saying that titans are not bad at all when you get to know them.
-The fact that they want to keep everything secret or otherwise they will get killed if they fail their mission.

Maybe the walls house different kind of humans before and they were all exterminated, maybe this happened a lot of times, one group of humans get inside and get comfy, get's exterminated in 5 to 15 years depend on how long it takes for titan shifters to destroy gates and when every last human is dead they kill of the titans inside, seal off the walls and let another group of human refuges flee inside for certain amount of time.
But this time the ritual was interrupted because of Eren, one titan shifter that came out of nowhere and spoiled their plans by closing the gate. It seems that their objective is not to kill him but to show him the outside world and the reason behind all of this.

Anyhow this is just a theory, you are welcome to post your own opinions and theories on this matter.

 

[kkck]

I don't think those 3 would have been aiming for a position. They spoke about their mission about something which necessarily had to be done and something which they utterly despised. I don't think the 3 of them would have gone on their mission for something so simple, I don't think they would have have the heard to go through with it under those circumstances. Whatever they are doing is of such importance that the potential deaths of all of humanity was deemed a necessary sacrifice. It wouldn't even be strange if we were talking about the survival of the warriors beyond the walls as a whole. There wouldn't be enough humans in the world to actually make such a test many people either.

 

[DarkLordOfKichiku]

I agree, the whole matter's a tad too complicated to be "just" a way for RBA to prove themselves as "Warriors". For that matter they speak of themselves as already being warriors, so presumably a "Warrior" is among them simply a Titan Shifter...

As for Ymir, I doubt she was ever a "Warrior", althrough she does seem to know a thing or two about RBA's people... For that matter, her talk with RBA implies that bringing them Historia won't actually save her, but might save Historia at the very least...

 

[Gourmetguy]

What if the human Titans in the wall are good and sacrificed themselves to protect the humans and make the wall.
They may have fought on the side of the humans 100 years ago. The prophecy suggests not to wake them as the walls will fall.
Perhaps these could be champion Titans who will assist the humans once more in the end.

I also think Annie, Bertholdt and Reiner are misguided and may have been brainwashed by the Ape Titan, who could be their leader.

Apart from Annie, Reiner and Bertholdt seem to have some compassion and haven't directly killed anyone. so i think they will turn good in the end.

 

[eton]

The thing is, wiping out all humans in the city would be a very easy thing with all three of them, BAR, working in concert. You could have titans swarming all through the city eating everyone in a matter of hours, and with their knowledge of the military, BAR could obliterate any attempts at resistance. Humans are basically hopelessly outmatched against BAR + regular titans swarming in through breaches in the wall.

So BAR's plan may initially have been to wipe out mankind, but for some reason, their plans changed, they infiltrated the military's ranks & lay low. Possibly they knew Eren would shift and they were waiting for that to happen before seizing him.

Why they initially wanted to destroy mankind, why they didn't just get on with it and do just that 5 years ago, why they want Eren, and who these 'warrior' people ARE, where they live, and how they've been able to live outside the city with the titans trying to eat them---all of that is a mystery still, but you can guarantee Eren's father is one of them, or has contact with them, or is some influential force.