In many, many works, when poison comes into play, it is the villain who primarily uses it, and indeed, use of poison by a character is seen as very suspect at best, and a strong sign that the character is evil at worst. Very few heroes (apart from antiheroic Combat Pragmatists) will use poison, considering it dishonest and dishonorable, and if they do use it, it's likely to be something aimed to knock people out such as tranquilizer darts rather than something deadly like arsenic or cyanide, which ties into Thou Shalt Not Kill. See also Trying to Catch Me Fighting Dirty and Poisoned Weapons.