TV is all about drama, and there are few plots more dramatic than planning a character's death by poisoning. While it might seem a bit medieval, there are plenty of TV shows set in modern times where poison is used to kill off characters. These TV characters are master poisoners, and quite ruthless as well.
Hannah McKay, "Dexter"
Most of the killing on "Dexter" is done with knives or bullets. But in the case of new character Hannah McKay, poison is the preferred method for murder. True, Hannah has killed people with weapons, as well, but her love of horticulture and botany means that she knows plenty of flowers and plants that can kill a person without leaving a trace on the coroner's report.
Hannah is believed to have used aconite to poison a former mentor, but also used rat poison to kill an abusive person in her past. She also killed investigative crime writer Sal Price by poisoning the outside of a pen: Sal had a bad habit of nibbling on pens during interviews.
Mags Bennett, "Justified"
In one memorable episode, Mags uses poisoned moonshine to kill off one of her "employees." The poison worked fast, killing the man in just minutes, but appeared to have no unusual smell or taste.
Poisoned alcohol is also featured in an early episode of "Revolution," where Maggie used poison whiskey to get a drop on a group of men who had taken her and Charlie hostage in an abandoned airfield.
Walter White, "Breaking Bad"
While Walt hasn't used the ricin poison that he created to kill anyone yet, it is impressive that he was able to prepare the poison in the first place. Originally prepared to kill Tuco, the ricin has nearly been used to kill both Lydia and Gus Fring. Who will it kill by the end of the show's finale season is unknown at this point.
Attorney Paulson, "CSI: NY"
In the second episode of the show's fifth season, the killer that the team needs to track down in a master poisoner. The method used to kill multiple people was a simple library book, the Tibetan Book of the Dead, the pages of which had been coated in a toxic, radioactive substance called thallium. The killer was revealed to be an attorney named Paulson, who killed his wife for the insurance money, and planted the radioactive book there to provide "proof" that her death was caused by her working conditions. The rest of the deaths were just collateral damage.