I think the difference between the three is an implied sense of heroism, honor, and self-defense. What do I mean by this?
In the case of the charging paladin, it is heroic for him to charge into mortal danger, risking his own life to slay those who could very easily slay him in return. To fight one's enemy face-to-face is considered the "honorable" thing to do and is therefore not frowned upon. The sorcerer puts himself at risk by being on a battlefield for he is weak and easily hurt without his magic to protect him. He risks almost as much as the noble Paladin. In both cases, the enemy can also see the danger and face it. They know what's coming and they can choose to fight or flee as they see fit.
The problem with the poison using rogue is that the enemy will never see him (if he's competant at all) and won't be able to defend themselves nor recognize the threat before it's too late. Also, the use of poison is not considered to be brave, noble, honorable or in any way self-sacrificing. It is "the cowards weapon" and that is, IMO, the reason why poison use is typically frowned upon.
Now, from a completely pragmatic point of view, dead is dead and the rest is irrelevant. The means with which you destroy the invading Orc horde, weather through glorious battle or dark trickery, doesn't mean squat as long as you get the same results. However, and I've stated this before, in the D&D alignment system Good does not equal Pragmatic. Therefore, the use of poison which is highly pragmatic is not considered a Good act. Neutral, sure. Evil, definitely. As far as the Lawful part is concerned, it depends entirely on your character. If he has no personal code preventing him from using poisons then great. Just beause you are Lawful doesn't mean you obey the laws of your society, it just means that you strictly follow some kind of personal code.