Prolix got it pretty well. I'll make an example. A few years ago, and ex-Russian spy was poisoned in a sushi bar in London by his ex-comrades, who were afraid he was leaking information. (these details might be wrong. These things get warped over time) Anyways, this man suffered for a few days, then died. Actually I think a sword fight would have been better for the spies. It wouldn't have made international news. Well, anyways, poison is cowardly, and can't be guarded agauinsy effectively. You waste away, painfully, rather than a quicker death by weapon. Poison can blind you, remove function, cripple and paralyze. And again, it is too sneaky, too dishonorable, too cowardly... Is that a description of a "good guy?" In other words, the inherent qualities of poison are exactly the ones that society disrespects, therefore tying them toother disrespected parts of society: the "Bad Guys."
Besides, "Good Guys" need to use massive machine guns that shoot flaming grenades, and knives so long that they need to be photoshopped to be put on TV, and he needs to drive tanks into kamikazi'ing helicopters with huge particle explosions in the background, and he needs to put a saddle on a nuke and ride it! And they need muscles! Lots of muscles! You can't have a wimpy "good guy," unless he's the nerdy comic relief side-kick of the protaginistic "Good Guy!"
I hope that clears up anything Prolix left vague.