Interesting subject this. Is poison evil? Well the allies (all of them) used chemical agents in WW1 and 2, though they are recognised as fighting on the side of the good. Vietnam - agent orange.
Having limits to poison use because of alignment seems a bit high handed. I know I would prefer rping solutions. For characters following dieties that do not approve - there's angering your god (not a wise move). For knights and the like there is the lose of face and respect that would bring. In roleplay terms other characters may not wish to associate with such types.
Their was a good point made with regards to the cultural significance of poisons. The types and the context of their uses. The city fathers of Hlint obviously don't dissaprove too strongly or the poison maker would be underground (thieves guild anyone?). There would no doubt be laws for it's use. Poisoning rats, goblins and their ilk would likely be fine.
Poisons used against citizens would likely result in a very harsh sentence or mob punishment (burn the witch).
All this has been said in the above comments really. The confusion about poison is appropriate though. Poisons, mental illness and witchcraft were interchangable to the general public.
I would allow poison for any character as long as they could roleplay it. Within the game they may find their actions scrutinized by all manner of groups (all types of rogues and ne'er to well, government officials, town watch, other players including those who see poison as evil) Having to prove their honest intentions to all and sundry and avoid the unwanted attentions of others could be a full time job.