The more I think about Lupin in DH, and at Grimmauld Place in particular, the more I feel like it was pretty in character. When Lupin was saying he regretted marrying Tonks, it wasn't because he didn't love her; he felt guilty about making her into an outcast. Lupin is a reserved person who doesn't cut himself a lot of slack - to feel guilty about impusively rushing into a marriage and unintentionally hurting the woman he loves isn't unlike Lupin at all.

Another thing that bothered me was the way Lupin seemed so manic about looking for a spy in the Order, but it's pretty obvious why Lupin, of all people, would be so obsessed with finding the rat. The mania almost made him look guilty himself... like he had something to hide, which is why he was checking all the others. But I think a lot of Lupin's attitude had to do with his involvement in the Order. During the two wars, when Voldemort was using werewolves to maim and kill, and Remus was living with those who had been shunned by society, he was always going to have a heightened awareness of his position as a 'dark creature'. For someone who's failing is wanting to be liked, other people's opinions are going to matter highly to him. It's not surprising that after being viewed a certain way for such a long time, Lupin would begin to feel guilty - like he was fooling no one but himself.

I never understood why during the first war, James and Sirius suspected Moony, and not Wormtail, to be the rat. I think it all goes back to Lupin's guilt. The Marauders have their flaws, but prejudice is not one of them. I never really liked the idea that Lupin was suspected because of his condition. After reading DH now I can see - Lupin probably wasn't acting like an innocent man. He's so worried about succumbing to the Death Eaters like other 'dark creatures' that he begins to act like he's already made the choice. Doesn't really excuse his actions, but at least gives some idea of what was driving him.