My opinoin is this.
Poison isn't honourable, its just another way of putting the other guy in the ground. An Orthodox Paladin waill not approve of poisons, because the way I view their code is they respect evil and will fight it fairly.
Which is rather odd, since that is what the code indicates and insunuates that evil isn't something which must be killed, more an enemy that must be defeated and do so with Honour.

The poison itself is evil depending on why you kill someone and who it is.