Doctor Who: The Doctor's 5 Darkest Ever Moments

The Tenth Doctor is arguably the fan-favourite Doctor of the modern era, yet despite this, he could also be incredibly arrogant. Believing that because he was the last of the Time Lords, he was entitled to control the Laws of Time. He allowed himself to become overconfident, meddling with a fixed point in time at Bowie Base One on Mars, potentially altering the future of the human race.

Initially deciding that he wasn't allowed to save the crew of the base, The Doctor changed his mind, and headed back to the colony in order to save them. Saving most of the crew, The Doctor returned them to Earth. Captain Brooke challenged him about fixed points in time, to which he responded by saying he was a "Time Lord Victorious."

Adelaide Brooke was forced to take her own life in order to ensure that the destiny of the human race remained on the correct path, and The Doctor realised this was the moment he had finally overstepped the mark and gone too far.