Afraid I have to disagree with you wilder_jw. I don't think the taboo on poison use is religious at all. Can't think of any religious texts (real or fantasy) that have anything to say on the subject. Historically in many cultures it seems to have been considered dishonorable though.
My guess would be the taboo is simaler to the Geneva convention ban on unjacketted lead bullets. It's something that might give you an edge, but the possibility of having it used on you makes you willing to give it up. After all, an unpoisoned arrow or a jacketted bullet will still probably stop an enemy, but a poisoned arrow or a soft lead bullet is more likely to kill or permanently cripple. So if everybody can agree not to use it everybody is better off.
Of course, poison is a lot less effective in D&D than IRL. Seems like they carried over the taboo without carrying over the deadliness. Oh well, that's the way they wrote the game.
Still I wouldn't say poison use is evil. Just likely to be frowned on by society.
Just my thoughts.
Tiew
My guess would be the taboo is simaler to the Geneva convention ban on unjacketted lead bullets. It's something that might give you an edge, but the possibility of having it used on you makes you willing to give it up. After all, an unpoisoned arrow or a jacketted bullet will still probably stop an enemy, but a poisoned arrow or a soft lead bullet is more likely to kill or permanently cripple. So if everybody can agree not to use it everybody is better off.
Of course, poison is a lot less effective in D&D than IRL. Seems like they carried over the taboo without carrying over the deadliness. Oh well, that's the way they wrote the game.

Just my thoughts.
Tiew