Poison's not evil. Just underhanded. The reason why a paladin avoids using poison is because it's essentially "dirty pool". But really, the main problem with poison is that it's a subtle and sneaky way of disabling or killing an opponent, often without even having to enter combat with that opponent. Contact and ingested poisons do nasty things if they're left lying around on food or on a sword's hilt. At least with spells you're standing and blasting someone, who has the opportunity for a counter-attack.
Using poison in combat is in essence enforcing a handicap on the enemy. You weaken your opponent, denying him a fair fight, so that you can beat him more easily. What's the difference between this and Ray of Enfeeblement? Semantics. The Ray applies a similar penalty, but does it without the baggage that goes with a poisoned knife. Besides, a wizard is supposed to be able to disable people with magic. It goes with the territory. But it also takes a certain special kind of person, who is already deserving of some respect. Any idiot can stab someone with a poisoned dagger, but magic is a pursuit that takes dedication and talent. Also, wizards don't have much else at their disposal for defeating foes, unlike a paladin.
So really, poison is considered undesirable because it's some kind of "coward's weapon," that throws off the results of an otherwise fair fight, or which is used to murder from a secret position. But in itself, it's just another kind of weapon, like magic, that hinders an opponent long enough to take him down.