I was thinking about moral defeasibility and moral defeaters today. In particular, I was thinking about the fact that, for instance, if A accuses B of pride and does so only out of pride himself, that this is a sort of moral defeat for the accusation. (Actually showing this is difficult, but I think it explains at least some uses of tu quoque in moral cases, and clearly it's an important phenomenon in moral terms.) A very good example is dishonesty: an accusation is undercut if the accusation itself is due to dishonesty (and this is so even if it is accidentally right). I'm not sure if all vices work this way. Some allow for this defeasibility very easily, like dishonesty; others are more complicated, and so I don't know if there are any vices that can't, under some circumstance, however rare, allow for this kind of defeat.
I've been trying to find if anyone else talks about the subject, with no luck; when people talk about 'moral defeaters' they usually just mean ordinary defeaters for arguments with moral content (i.e., they are ordinary arguments), which is a waste of a good label. These, however, are properly moral defeaters -- identifying dishonesty in and of itself undercuts any claims in which it is too closely involved, due to the way the vice affects reasoning. This is an important part of how people argue, so I would have expected there to be something about it. I would have expected virtue epistemologists to say something about this, but I haven't been able to find anything. If anyone knows of anything, let me know.