IMHO, Poison is like any other tool or weapon- whether its use is evil or good depends upon the circumstances surrounding the use.
Examples:
Evil use: Poisoning a village well.
Good use: Poisoning a BBEG who can only be truly defeated by use of a particular poison (IOW, the BBEG has a Tarrasque-like constitution and vulnerability); poisoning the food of an Army of Evil with an emetic or soporific in order to escape or to facilitate surveilance/infiltration without killing someone.
Evil use: using a poison that's sole effect is causing debilitating pain. Platypus poison, for example, isn't inherently life threatening, but it is long lasting and has 2 primary effects: it causes intense pain and it increases the ability of the afflicted to feel pain. The result is someone writhing in pain and screaming...for days.
Good use: using a non-lethal poison to defend oneself- a skunk (or Trog's) repellent scent is a form of poison; an inflammatory, sleeping or paralytic poison (that affects voluntary, not involuntary, muscle contractions) may be used to facilitate escape, or the live capture of a fugitive.
Paladins (to my thought) consider poison to be dishonorable...
Not all Paladins have an overdeveloped sense of "honor." Some might be quite pragmatic. Honorable ≠ Good, Dishonorable ≠ Evil.
For instance, is it for the greater good to confont the BBEG after raising an army to offset the BBEG's, thus fighting the epic War to end Evil, but resulting in the deaths of thousands of combatants and noncombatants on both sides...
or to infiltrate the BBEG's camp (never doing an overtly evil act, nor omitting a potential good one) and dose the BBEG insensate, carrying him off, and presenting him bound & gagged to the priests of the Church of Tyr (or St. Cuthbert, or whomever...)?