David Tennant's Tenth Doctor just committed genocide in Doctor Who's epic "Time Lord Victorious" transmedia adventure. Whether he likes it or not, the Doctor is frequently put in a position where he has to decide the fate of entire races. Who will live or who will die? So many times, it's been up to him.
The most famous example was in "Genesis of the Daleks," when the Time Lords sent the Doctor back to the dawn of the Daleks and presented him with the opportunity to wipe them out. He chose not to, concluding he did not have the right. "I know that although the Daleks will create havoc and destruction for millions of years," the Doctor insisted, "I know also that out of their evil must come something good." It was a fateful decision, and one he came to regret; Big Finish's recent audio-book "Out of Time" featured a meeting between the Fourth and Tenth Doctors, with the latter insisting he not only had the right to kill the Daleks, he had the responsibility to do so.
"Out of Time" was teased as something of a prologue to the "Time Lord Victorious" arc, and it's easy to see why - because the Doctor has just followed that same chain of logic, committing an act of genocide and changing the entire timeline. In Steve Cole's novel The Knight, The Fool And The Dead, the Tenth Doctor travels back to the Dark Times when the universe was young. There, he learns an alien race called the Kotturuh are the ones who introduced death to the cosmos, and he resolves not to accept it. Instead, the Doctor learns to turn the Kotturuh's power back on them. The immortal race who believed they had the right to judge all life are themselves judged, and wiped out.
While the Kotturuh are hardly portrayed in a sympathetic light, the fact remains the Doctor has crossed a line. In a logical extension of his "Time Lord Victorious" arc in "The Waters of Mars," the Doctor essentially declares himself a God, and one with the right to shape all of creation. "The grass withers and the flowers fall," he declares, paraphrasing the Bible, "but the word of a Time Lord stands firm forever." It's a phenomenal act of hubris, and the repercussions are driving the plot of "Time Lord Victorious," as other Doctors find themselves living in a universe in flux.
The Knight, The Fool And The Dead ends with a tantalizing closing scene, one in which two other incarnations of the Doctor arrive to prevent the Tenth Doctor destroying death forever. The Kotturuh have died, and three Doctors are about to go to war over the nexus of death itself. The scale of "Time Lord Victorious" is truly breathtaking, with all of creation at stake.