Every hero needs an arch-villain, The Doctor just happens to have two. One of which is the titular creature in this episode, whos fallen to Earth after the previously mentioned Time War. The Ninth Doctor was as hard nailed and bitter as he was because this Time War wiped out his race, as well as the Dalek race. Faced with the unthinkable, The Doctor had to kill his own kind in order to prevent his enemys victory. Though as well learn later, this isnt as clear cut of a story as we think. The beauty of this episode is that were introduced to an Old School villain, but a new (and mostly more sympathetic) spin is put on this genocidal race. Even better, we see some of the darker points to the Doctors psyche, as the man who abhors guns and killing has no problem letting this creature die. The Doctor is like any other hero in the respect that he can hold grudges. He can want his arch-nemesis to die and suffer while doing so, but he doesnt have to be proud about it. Ultimately, its his companion Rose who helps hold him back from being a completely murderous maniac; a trait well soon come to see as priority one for any future companions.