In chivalric circles, poison was defined as a coward's weapon.
In 1e RAW poison use is explicitly noted as evil, which is nice and simple and therefore to my liking.Only Evil Clerics and all Assassins are freely allowed to use it, Paladins and non-Evil Clerics are banned from it, and for all other classes it's left open as - in part - an alignment definer.
I can understand that it's cowardly.
But why is it evil? Or, at least, any more evil than burning someone to death, reducing their mind to a vegetable, etc.