The Uncultured Anonymous/Poisoning
Poisoning is not an overt act of offense or defense. Generally, it is used by assassins to kill people in a less overt way that is subtle and they hope won't be caught. Police know about poisoning though, so it usually doesn't work, even if you don't use obvious poisons and just give someone more medication than they're supposed to have, they'll still catch you, just like on CSI.
Poisoning is also done for reasons besides murder, such as sabotage or lulz. For example, if one were to poison someone, they might fail at the Olympic competition and you win the gold for your country. Or, it might be funny to see a guy vomit up toxins while out on an anniversary dinner with your ex-girlfriend. Even if your intent is not to kill, this could happen accidentally, and even if it didn't, you can still be charged with the police with attempted murder for it if that's what they think (or just if they don't like you and think the charges will stick).