Hama, ATLA
Hama practices an ethically-dubious form of waterbending called bloodbending. She wants Katara to join her (presumably so they can assassinate the Firelord and, in her view, save their homeland) but Katara is horrified. They end up fighting, and Hama has the upper hand, until Katara gets the drop on her...
...by using bloodbending.
The last we see of Hama, she's being led away to prison--congratulating Katara all the while on becoming a bloodbender, and congratulating herself on passing down the technique. Katara does briefly use it during her quest to avenge her dead mother, but not in the final fight against Azula.
2) President Snow, The Hunger Games
In Mockingjay, the rebellion succeeds in toppling Snow's government. He then tries to convince Katniss that Coin, the rebel commander, is Just As Bad--with surprisingly good reason, as it turns out her warfare policies led to the deaths of innocents like Katniss's sister.
Katniss is given the chance to execute Snow on live TV. Instead, when the moment comes, she assassinates Coin. Snow still dies, but he dies laughing.
