Well, I've been to lazy to reread book 5 like I said I would, but here's my take on it.
I don't think that Lupin really mind's being a werewolf. Remember his teen wolf days were some of the best days he's ever had. And remember how Lupin comforted the werewolf guy at the hospital? He was telling him about how it wasn't all that bad, and that it is quite easy for a werewolf to live a completly normal life. I mean, it's only once a month that he becomes a monster, and with Snape's wolfbane, he's not even a monster
then really.
And werewolves are just so bada$$
. I think the only real downside to being a werewolf would be the preconceptions and judgements that society in general would make about you. I think that Lupin's ethical character is too high to really care about what a bunch of small-minded, bigoted, idiots might think about him, much less kill his best friend.
I believe that
if Sirius was betrayed, Moody may be a posibility. We don't really know anything about the guy and pehaps the evil "Moody Imposter" was in book 4 to foreshaddow Moody's potential evilness. As for someone who could counter Sirius's skill, Moody was a powerful Aura, and even so he got hurt pretty badly in return. (I believe that he was knocked to the floor and his eyeball rolled around for a bit.) So even if Moody
did betray Sirius, at least Sirius got a powerful blow in.