Well, I think R&B are an interesting couple indeed (they are totally into each other btw). Those two has been through some pretty nasty sheit.
They probably were raised as fundamentalists (remember their flashbacks, espcialy the one about Annie's father), have witnessed horror themselves (Ymir devouring a friend of theirs and Titans attacking their village). Also, they had to (were forced into?) go on a very dangerous mission among enemy lines and god only knows what else.
I find Reiner to be the most interesting of the two. I really believe he is sincere in all instances. He's a warrior, proud of it, and with a strong sense of duty. He values determination and bravery onto anything else (to the point of stubborness), and immediately respects Eren as soon as he hears his whole story. He believes in such high and noble ideals that he suffers a fierce internal conflict when getting to know the (so viscerally hated) enemy better, causing his split personality. He is really ready to put his life on the line when saving Connie, would do the same for all of his Soldier companions, and would undoubtedly do the same for Berth. Regardless, he knows what he did was bad and unforgivable (even more so because he got to know not every human is as ugly as he once thought), surely feels guilt about it, but will be carry his duty no regardless such for the sake of being a warrior.
I believe Bertholdt has been true to himself too right from the start. It's just he doesn't really seem to believe in what he's doing as much as Reiner is. If I would have to guess, I'd say Bertholdt is pretty content with doing whatever he's told to. After all, he is both a coward and lack any self confidence. By doing someone else's bidding, he doesn't even have to bother believing in what he's told to believe into (and, by doing so, feeling perfectly fine with it). By doing so, he would always be able to just state "it's what they told me to do", in fact he also admits he doesn't really consider himself responsible for whatever he is doing ("I don't have a will of my own"). He doesn't seem to really give a shit about anyone but Reiner. It's not like he doesn't realize how many (innocent?) people he has killed, but he doesn't feel guilty for it in the slightest, and he never reacts in an emotional way even when witnessing the imminent death of his comrades. He's not even able to sympathize much with Eren when hearing about the facts he's been thorugh. When he says "I felt sorry for you" regarding Eren's mother he's probably true, but you can tell from his face he doesn't feel any guilt, just like it wasn't his fault (and in his mind, it really isn't).
So in a sense yes, I think you could say Berth has a somewhat neutral morality. He does realize killing people is bad I suppose, but at the same time he's willing to do so because that means he doesn't have to take responsibility for whatever decision he might take. He rather prefers to have someone else take decisions in his stead.
