After re-reading the manga chapters, I've noticed that Annie, Reiner and Berthold refer to themselves and each other as 'warrior'
When Annie finds out that the military discovered her true identity, she says that she's a 'failure of a warrior'.
In chapter 39, Berthold refers to Reiner's true identity, when answering Connie's question 'Was Reiner always like that?', by saying that Reiner 'used to be a warrior, different from who he is now' (or something like that).
Finally, upon revealing his and Berthold's true identities in chapter 42, Reiner says that 'as a warrior, I gotta take responsability and fulfill my duty till the bitter end'.
According to Wikipedia, a 'warrior' is a person who is 'engaged or experienced in warfare'.
Now I could be wrong, but here's my analysis of this:
The use of the word 'warrior' and other military language such as 'fulfilling one's duty' hints at the society where Annie, Berthold and Reiner come from putting a lot of importance into warfare or, even more, being a military society where one's is judged based on his ability to fight.
From my understanding of human history, such military societies mostly existed in ancient times, back when small societies were rapidly expanding and constantly fighting each other for territory, natural resources and women. In other words, they were military societies out of necessity.
Now we don't know much about Reiner's, Bertholds and Annie's hometown yet, but if they come from the kind of military society that I think they do, it could mean that they are trained for combat from a young age in order to protect themselves from a great threat. It could also mean that the 'Titan's power' has been created or at least used by their people in order to protect themselves from that same threat. We know that the Titan's power was stolen by Ymir who then used it on herself, so it has almost certainly been created by someone for a greater purpose. We just don't know by who and for what purpose.
Now this is all speculation, but I think Ymir is right when she said that killing Reiner and Berthold would not put an end to the people within the walls' trouble. Historically, training their people from a young age so they can learn how to fight using a power such the Titan's one, isn't something that societies do just for fun. It's usually so they can fight or protect themselves from a powerful enemy. And I doubt that enemy is the innocent people within the walls.
Exellent analysis. For some reasom it made me think of the Spartan society. But I think that there is more to this society. Who is to say that there is only one warrior faction? In comparison to RBA the Ape titan seemed to know little about what is going on in the human world.
I don't think Reiner and the other's motives are purely malicious. Burtholdt seemed to be pretty guilt ridden when confronted by Jean and the others and Reiner is actually experiencing disassociation, most likely to irreconcilable guilt as well. This combined with the fact that the priest seemed to consider the secret of the walls more important than the lives of the people living within them seems to point to the fact that there is a much bigger picture that we're not seeing just yet. I think Irvine has a clue as to what's going on because of his mysterious "who do you think the real enemy is" line.