I think that poison is chaotic.
Hence, the reason it is often against the law.
It can indiscriminately injure or kill anyone who comes in contact with it.
Hence, it would typically not be evil for a Paladin to use it, it would be chaotic of him to use it.
In the real world, lower powered poisons (e.g. rat poison or insect poison) are legal, but their use is typically restricted to killing vermin. Hence, they are legal with a caveat. Higher powered poisons are illegal due to their danger factor.
I would think that the same would apply in a fantasy world. Poison would be chaotic and would only be used in legal society under certain guidelines. For example, some low powered poisons (e.g. lye) would probably be used by commoners to take care of vermin infesting their livestock or to assist in everyday tasks (like preventing pit latrines from blowing up). Although we as players would consider it poison, the NPC commoners and their lords in the game might not consider it as such. Alchemists might have a great deal of what are considered poisons, but their services might allow them to legally use them to perform specific tasks.
Other poisons (i.e. poison in peoples food or on weapons) should in most fantasy societies be considered illegal. So, potency and use might be the two factors to determine whether a substance is legal or not.
With regard to evil, our real world society considers using poison indiscriminately in combat to be immoral (e.g. mustard gas) if the poison is lethal, especially if there is a lot of suffering involved and the time it takes to die is lengthy (e.g. mustard gas often takes 4 or 5 weeks to kill someone). However, if the poison is merely mildly harmful or annoying (e.g. tear gas), it is considered moral to use it.
In a real world setting, poison is neither chaotic nor evil. It is an inert substance. The use of it is what is evil.
However, in a fantasy setting where the Gods of Oceans and Storms and Fire are often chaotic, I consider poison (and rivers and oceans where people can indiscriminately drown, or fire where people can be burnt) to be chaotic elements of nature.
So, poison is chaotic in a fantasy setting (IMO), the more powerful the poison, the more chaotic. Hence, using poison to attack someone is often but not always, a chaotic act just like using fire to attack someone is often but not always, a chaotic act (since fires can easily get out of control). Just like firing a bow indiscriminately into a crowd of combatants (both allies and enemies) would be considered a chaotic act (at least in my game where there is a fair chance of hitting anyone in the area if you miss the target).
But, the use of poison can also be evil depending on circumstances in a fantasy setting, just like in real life.