"Poison use is evil," is a blanket statement that is a derivative of the actual rules and isn't always true.
The actual rule put foward by RAW is (paraphrased) "Using poison that causes ability damage is evil if it's not a natural ability of the character using it."
If you have a poison bite, poisoning your enemies with your bite isn't evil. If you put knockout poison on your blade it isn't evil. If you use that poison that lowers spell resistance on your crossbow bolts it's not evil.
Btw, here's something to consider about the excessive suffering caused by poison that doesn't apply to hp damage.
The problem lies in the assumption that the enemy will be killed by the poison, only a handful of all the available poisons cause con damage, or immediately after being poisoned.
Suppose you poison an enemy, that either goes on to kill you anyway, escapes, or survives because reinforcements drive you away. If he failed the initial fort save, he's already been partially crippled, and will take several days to recover his full ability. He also has to make another fort save in the near future if he hasn't already and may be further crippled, paralyzed, or even rendered comatose. He won't be back to his normal self for days, perhaps even as long as a week or more, barring magical healing which may or may not be available.
All this is assuming that if he was rendered helpless for an entire day, he wasn't left so in the wilderness where he'd be exposed to the elements, and any scavengers that might think he looks tastey, which could amount to a slow aggonizing death from exposure to extremes of weather, or being eaten alive by a scavenger that thought he was dead.
HP damage doesn't do any of that. A character rendered unconcious by HP loss will almost certainly die within a minute, unless he recieves immediate attention or has uncanny luck.
In the case of him simply having uncannily bad luck he could end up starving in the wilderness, being eaten alive, or being exposed to extreme weather, but that's because of sheer dumb luck (made the stablization check, then failed the fort saves to avoid exposure, starvation, or even simply the check to regain conciousness) not because you intentionally and knowingly doomed him to days of being crippled or perhaps a lingering death.
If he survived through the battle without be dropped below zero HP, the HP damage won't impede his ability to do anything except survive further attacks.
Before anybody tires to counter with, "what about ravages," consider that they are A)magical in nature, which means they're already playing by a different section of the rules, B) affect creatures normally immune to poison, which differentiates them further still, and C) work only on evil characters, when BoED and BoVD both give leeway to pre-emptive strikes.
Mind, that's pre-emptive strike, not provocation. You're supposed to have a legitimate reason to harm the creature before you use any kind of attack agaisnt it, ravage, poison, sword, spell or other.
The same logic follows for disease and afflictions. Disease is damning a character to perhaps months of ongoing ability damage and/or a very lingering death, while you need a spell and a valid target, that you're supposed to have already properly vetted, to even use afflictions.
As long as you told the villian what the affliction was when you hit him, or he killed you before you had the chance, he's responsible for anyone he spreads it to, and he can cure himself by changeing his alignment and ceasing to be a valid target for the affliction.