The Tenth Doctor believed that he was the last of the Time Lords and subsequently decided that the rules of time no longer applied to him. Declaring himself as the "Time Lord Victorious", he assumed that he had sole authority over time itself and would use his power to save influential figures throughout history, as well as the normal everyday folk, whom he labelled "the little people". After saving Adelaide Brooke from her death on Mars, he saw that he was justified in making this decision, as he changed the timeline to what he thought was for the better. After she decided that it wasn't his decision to make, however, she took her own life and the ramifications of his actions finally began to sink in. And it was about time, too! That's what you get for being cocky, Doctor.