Mercy

According to the Seventh and Twelfth Doctor, mercy was a word which should not exist in a Dalek's vocabulary. (COMIC: Nemesis of the Daleks, TV: The Witch's Familiar)

Facing River Song, a Stone Dalek cited its records of the Doctor's associates which indicated that she would show mercy. However, once River Song revealed her identity, the Stone Dalek repeatedly begged, at her command, for mercy before it died. (TV: The Big Bang)

During the Last Great Time War, both the Daleks and the Time Lords were generally thought to have no mercy for those caught in the crossfire. (AUDIO: The Famished Lands)