No. Not inherently evil. Just like a sword or gun is not inherently evil. It is the old clique - Guns Don't Kill People - People kill People.
IMHO, there is not much difference between gut stabbing a foe with a sword and having him die a painful death or poisoning a foe and having him die a painful death. The outcome - dying a painful death is the same, only the methodology differs.
So, why has poison got a bad rap? Because it is not considered honorable. Facing a foe with a sword who is likewise armed is considered honorable combat because you are at equal risk as your foe for death or injury - a level playing field so to speak.
Poison, on the other hand, allows one to kill without risk. Kill off your foe by having some minion poison his wine or food. Even a poison weapon is not considered honorable since you are considered 'stacking the deck' in your favor - your weapon has an advantage by making a minor wound potentially lethal due to the poison. Your foe, with armed with an unpoisoned similar weapon, does not have such an advantage.
Therefore, the use of poison is not evil but has moral implications pertaining to honor and fair play. As such, Paladins and others who subscribe to some honor code of what is considered 'fair play' or 'honorable' combat are going to frown on it.