According to the footnote on the bottom of page 80 in the DMG, "The purchase and possession of poison is always illegal...."
That certainly seems to mean it's an unlawful thing to use poison. Of course, not all laws are applicable to lawful characters, as some of them follow their own code of behavior.
I personally believe poison use isn't a lawful *or* good act because it's indiscriminate. If you leave a sword lying around, someone has to pick it up and use it to cause injury to another, willfully and consciously using the weapon.
The same is not true for poison. It's very existence is a danger to innocents as it can easily be misused or mishandled. If a poisoned drink isn't consumed by the target and instead a small child drinks it, the poisoner is responsible for the death of an unrelated target.
While a case could be made that firing an arrow into a crowded room of enemies is a similar indiscrimate attack, there is an element of precision to the use of the bow. If the arrow misses the target, it's resolution is fairly instantaneous. If something goes amiss, the consequences are generally immediately evident and can be resolved right away. Poison continues to linger as a danger, possibly for years.
I can't see any lawful character using poison.
Good characters likewise should be concerned about the possibility of poison later being a threat to the innocent.
Greg