Murder

Murdering a being could result in numerous penalties. Common ones were life imprisonment and execution. (TV: The New World) Killing beings on a battlefield did not usually count as murder. (TV: The Caretaker) A killing classified as murder usually meant it was a targeted attack on a single individual. Serial killers would target multiple people over a period of time. (TV: They Keep Killing Suzie) Murder of an entire race was called genocide. It was one of the most serious crimes in the universe. (TV: Evolution of the Daleks, The Doctor's Daughter, Journey's End)

In the 51st century, murder was seen in a less serious light. Unlike most cultures around the 21st century, murder was considered an addiction like drugs and alcohol and instead of a death penalty or imprisonment, the murderers would be sent to rehab. (TV: Kiss Kiss, Bang Bang)

Morality
The Seventh Doctor felt that, fundamentally, the only argument against murder that could stand up was utilitarian: "we don't [ourselves] want to get murdered." (PROSE: Original Sin) ""In the end, Pryce was right. There is no reason why one person should not kill another. No argument against murder stands up to scrutiny. For every religious prohibition saying, "Thou shalt not kill", there's another one that allows killing under certain special circumstances – sinners are fair game, "Thou shalt not suffer a witch to live", and so on. Moral codes are no better; they're just formalised opinions, without any logical backup. The sociological history of almost every race is riddled with examples of laws against murder standing beside legalized examples of murder, be they executions, wars or euthanasia. Ultimately, every single argument that we can come up with, stripped of its pretty words, boils down to a fundamental truth: we disagree with murder because we don't want to be murdered. No more and no less than that.""

- Seventh Doctor

The Doctor
On several occasions, the Doctor often battled over whether they "had the right", to take another person's life, even toward altruistic ends. (TV: Genesis of the Daleks, The Parting of the Ways, The Witch's Familiar) In some cases, they took this option. (TV: The Wheel in Space, The Seeds of Death, Day of the Daleks, The Talons of Weng-Chiang, State of Decay, Earthshock, Arc of Infinity, Resurrection of the Daleks, Planet of Fire, Attack of the Cybermen, The Two Doctors, Terror of the Vervoids, Remembrance of the Daleks, The Runaway Bride, Dinosaurs on a Spaceship, Deep Breath, The Doctor Falls; COMIC: Master of Spiders, Terrorformer)

The Doctor counted the number of children on Gallifrey that died at the end of the Time War, when he destroyed the planet. The Tenth Doctor knew this number well, as 2.47 billion, but by his eleventh incarnation, he had forgotten this. The Eleventh Doctor expressed that he had "moved on". (TV: The Day of the Doctor)

By his twelfth incarnation, the Doctor did not keep track of how many people he had killed, as he felt the need to move on and save other people's lives outweighed an obligation to honour those he had failed. (TV: Thin Ice)

Examples of murder
In 1893, the serial killer Clarence DeMarco was due to be executed for murdering fourteen women. He was spared due to his information on the Doctor's secret. (TV: The Name of the Doctor)

In December 1916, the Russian mystic Grigori Rasputin was murdered by Prince Felix Yusupov and his co-conspirators Grand Duke Dmitri Pavlovich and Vladimir Purishkevich in the Moika Palace, Prince Felix's residence, in St Petersburg. (PROSE: The Wages of Sin)

On 11 June 1925, George Cranleigh murdered several servants at Cranleigh Hall in Oxfordshire. The Fifth Doctor was accused of having committed these murders. (TV: Black Orchid)

In June 1926, Professor Gerald Peach, Miss Chandrakala and Roger Curbishley were murdered by the Vespiform Reverend Arnold Golightly. He did so in the style of Agatha Christie's novels. (TV: The Unicorn and the Wasp)

The Master murdered untold numbers of people over their lifetimes. Some of their victims included Tegan Jovanka's aunt Vanessa and a policeman when they found his TARDIS in London on 28 February 1981. (TV: Logopolis) Another of the Master's victims was Thomas Milligan, whom he murdered with his laser screwdriver in an alternate timeline in 2008. (TV: Last of the Time Lords)

In 1985, Shockeye of the Quawncing Grig murdered Oscar Botcherby when he wouldn't accept a twenty narg note for payment of his restaurant bill. The Sixth Doctor later murdered Shockeye with cyanide. (TV: The Two Doctors)

Unusual and unexplained murders often occurred on and around the land that the Hawthorne clinic was built on, dating back to the 13th century. This was as a result of its proximity to the reservoir of evil. Daniel O'Kane murdered his mother, father and sisters in 1945 and, through supernatural means, numerous people including Clegg, William Bruffin and Colin Dove in 1993 and 1994. (HOMEVID: The Zero Imperative)

Oswald Danes was a convicted murderer, rapist and paedophile. In 2006, he raped and murdered 12-year-old Susie Cabina. (TV: The New World)

On 25 December 2006, the Tenth Doctor accused Harriet Jones of mass murder when she ordered Torchwood to shoot down the escaping ship full of Sycorax. (TV: The Christmas Invasion)

In 2008, Lucy Saxon murdered the Master with a gun. (TV: Last of the Time Lords)

River Song was convicted in the 52nd century of murdering the Eleventh Doctor at Lake Silencio on 22 April 2011, but the Doctor faked his death using a Teselecta double, of which she was aware. She was sentenced to twelve thousand consecutive life sentences for the crime, but was later pardoned after the Doctor removed himself from all databases in the universe, erasing all evidence of his existence. (TV: Flesh and Stone, The Wedding of River Song, The Angels Take Manhattan)

On 1 January 2019, Lin murdered two police officers and stole their car. (TV: Resolution)

The Fourth Doctor murdered Mehendri Solon by injecting cyanide gas into his laboratory. (TV: The Brain of Morbius)

In 2367, the Eleventh Doctor murdered Solomon by placing a beacon (being tracked by missiles) on Solomon's spaceship. The missiles made contact with his spaceship moments later, killing Solomon. (TV: Dinosaurs on a Spaceship)

In 2472, Captain Dent murdered Eric and Jane Leeson using an IMC robot while using a visual trick to shift the blame on a giant lizard. (TV: Colony in Space)

In 4126, Klineman Halpen murdered Dr Ryder by throwing him into the Ood Brain. (TV: Planet of the Ood)

A Dalek, pretending to help humans of the Dalek Foundation, murdered some survivors of a train crash and the medics tending to them while there were no witnesses. (PROSE: The Dalek Generation)

In the year 50,000, the bounty hunter Ahab murdered Zanthus Pia. He tried shifting the blame onto K9. (TV: The Bounty Hunter)

In the far future, Sarah Jane Smith declared Styre a murderer after he shot down a retreating Roth. (TV: The Sontaran Experiment)

References to murder
In an introduction that wrote for Doctor Who Magazine, he admitted that his life as the megalomaniac's megalomaniac wasn't all "murder, miniaturisation, and mercenary acts", and did have acts of so-called "charity work", which often involved more murder. (PROSE: A letter from the Master)