Poison in 3.x
Poison has generally been 'considered' evil due to lingering ideas about 'honour' from the middle ages when it wasn't in the interest of the people in power to allow the use of poison (and assassination in general) to go unpunished.
This is a bit of a poser for dnd (and other genres of entertainment) trying to incorporate the 'cunning hero' archetype into their piece of fiction. Having Poison Use be a special ability you need to be an assassin or Red Mantis Legionnaire or whatever with magical powers and being an 11th level character to be able to put some rat poison in someone's stew without accidentally eating some is largely due to this conflict.
Thus various authors have used 'poison' as an example of an evil act in the same game where Rogues regularly 'sneak attack' their foes, and player characters as small groups fighting larger ones entirely rely on confusion, stealth, and guerilla tactics, divorcing some aspects of 'cunning' warfare from others to legitimatize it as a hero type without going too far into murky waters. Similarly, the playerbase (and gmbase) has latched onto that and accepted it as read that while poison is evil, knifing a goblin sentry through the throat from behind is just fine.
Putting poison into someone's food can be either good or evil. A captive princess putting poison into the food of an evil, psychotic warlord knowing that his followers will kill her in the most horrible ways for her act, even as it saves countless others, is good. Exalted, even. A jealous brother putting poison into the food of his older sibling, in order to steal a princedom, is evil. It could even be neutral - making someone sick so they won't go to war, because you care about them.
Basically, there is no direct RAW indication that using poison is always evil in pathfinder, and even if there was, you should ignore it. Alignment is explicitly - and has always been explicitly - something the GM needs to decide based on the individual situation of the action.