Doctor Who: 10 Times The Doctor Acted Completely Out Of Character For No Reason
9. Taking Pleasure In Winning The Time War
There are a lot of justifications that can be made for the end of The Water of Mars, where the Doctor, shortly after doubling down on his destruction of Gallifrey by sending Rassilon & Co "back into hell" in The End of Time, abruptly decides that he is the victor of the Time War and can jolly well do whatever he likes.
Those justifications are thoroughly examined in Russell T Davies' own book, The Writer's Tale: The Final Chapter, but to save everyone the effort of reading such a big book, they can be boiled down to "I thought it would be good TV".
Taking the Doctor down a dark path where he begins to do morally questionable things because he thinks he's in the right is, undeniably, a great narrative idea. But what viewers got at the end of The Waters of Mars was a hastily presented and sadly jarring nosedive into the "dark side" of our favourite Time Lord.
If the character development had been given more foreshadowing, it could have worked. If the change to the Doctor's personality had been stretched out over multiple stories, it could have worked. Shoe-horning it into the final act of one adventure didn't work.