Socraties, the great greek philosopher, decided to perform his own execution by hemlock. This was not an evil act, as he was simply choosing his method of death and the trial was already over. In this case, the
trial makes the use of poison legitimate. Governments have always been allowed to susspend laws, like "no killing" to do what it thinks is neccessary, like execution. The state has that right because they are in charge and official. Because most characters are not government officials with the right to susspend the law they can not use poison.
As far as the orc camp example, what if you poison the water, and the orcs get to their home the next day and water makes the whole tribe sick. Now you haven't really punished the guilty raiding party, because they are more likely to survive, but you have hurt the innocent: the women and children who stayed back at the camp. Also, what if the orcs discover the poison water and dump it into a stream. Where are you then? This is why poison and biological warfar has been outlawed. It is a horrible way to die and you can't control it.
In "Enemies and Allies" book includes a smuggler character who is identified with the following statement:
Poison is a coward's weapon, and only the worst sort of person would use it. I'll have a shipment in next week, but get here early as it goes fast.
As for the poison and contaign spells, they can be used as a weapon with no chance accidental use. "My church chooses to punish you by inflicting red ache on you." This is still an "official" kind of use, just by a church rather than a government. Also, this "poison" is lying around for others to use. I would call the use of contaign in combat evil. You are just trying to spite your enemy, not defeat him.