Article 7

Article 7 of Gallifreyan law prohibited Time Lords from committing any form of genocide, with no exceptions. While the Sixth Doctor was on trial for interfering in the affairs of other peoples and planets, the Valeyard accused him of genocide for destroying the Vervoids aboard the Hyperion III. Violation of Article 7 accords the perpetrator a death sentence, held as a far greater crime than transgression of the non-interference policy. After defeating the Valeyard in the the Matrix, all charges against him were dropped. (TV: Terror of the Vervoids, The Ultimate Foe)

In a parallel universe, article seven of Gallifreyan law forbade state-authorised genocide. It dated back to the Vampire Wars and was countersigned by Rassilon himself. (PROSE: The Quantum Archangel)
 * How much of this corresponds to the prime universe is unclear.