IMCs, using poison is considered generally evil.
Basically, it's going out of your way to do harm to someone. It may not kill them outright but that doesn't make it right. There is a reason why bounty hunting is frowned upon, in general. Poison usage is not always evil, but there is a fine line. When I think of poison use, I don't think of jabbing someone with a poison dart and them just falling asleep or somesuch. More often then not, the person making the poison really doesn't care if it causes pain or any other kind of physical damage (twitching, spewing, bleeding, etc.).
And if someone takes ability damage, that is a direct statistical representation of that character's loss of function in some area. If I take DEX damage, that means I can't control my limbs (or some other part of my body). If I take STR damage, my muscles and bones are most likely wracked with pain or simply drained of strength; that can't feel good.
And as for the arguement (which has been used on me many times) of "well, I'm just going to kill him anyway it doesn't matter how" I say that's bunk. Intent to kill is different than an intent to maim, cause unneccessary pain, cause extra pain, or torture then kill. These are examples of gratuitous and cruel acts rather then just killing. Most goodly PC's only kill things that are evil or things that are trying to kill them. These are either acts of self-defense or defense of some kind. If PC's go about indiscriminately killing things, that is still wrong and is considered evil when I DM.
So unless someone specifically makes the poison so it is painless and meant to temporarily and safely (like for medical uses) incapacitate others, I consider it evil. It's not an evil act every time, but if a PC constantly uses poisons that he doesn't make, eventually there will be one that causes significant damage to an opponent, so that's his one chance to stop. One you see someone in obvious pain resulting from something that you have done that means that any subsequent actions are unforgivable because the character knows he is doing wrong...
EDIT: But as for using poison for good, the character would have to make certain he was doing as little harm or causing as little pain as possible. It is possible with a high enough skill in it. Doctors do it all the time. Of course they have access to much better technology.
