All right, that's fair - I think that approach leads to something like like, country A should not do morally-questionable action M, nor should country B, and if country A chooses to do it anyway, this shouldn't justify country B saying "Well why can't we" - country B should stick to its position and criticize country A. I'd agree with that.

(I originally read your comment as saying that it is unfair for country B to be enjoined from action M if country A is engaging in it, and I think other commenters did too. I do absolutely agree with the goal of collectively constructing a better good in a fair and equal way, holding everyone to that standard, and expecting that people will fail but it's worth trying to meet that standard nonetheless.)