A NAP is just to say an other player that you have no interest in attacking him, and that you are strong enough to crush him even when he is prepared. (Or that he could crush you if he wanted so, and you just hope that he agrees and you can live on
)
I personally do not backstab on NAPs, but if people do this i consider it their right (but i will note the player, and never trust him again). I think that casting Anonymour Rituals on nations you have a nap with can make a lot sense, and while the grace period is running down, even not so anonymous artillery might be used.