Whatever is done for love always occurs beyond good and evil.
In Dalek, Rose and the Doctor find themselves in a museum run by a rich collector of all things alien. Among the “objects” in this museum is the last of his kind–the sole surviving Dalek of the Time Wars. This Dalek is kept under deplorable conditions and is tortured regularly in an attempt to force him to communicate with the collector and his employees, none of whom understand the nature of the being in their possession.
Daleks, as a race, were notorious destroyers of worlds. During the Time Wars, they fell into such conflict with the Time Lords (the Doctor’s race) that they very nearly ripped a hole in the universe, forcing the Doctor to partition that area of space from the rest of time and essentially destroying both races. The Doctor was the Survivor. Apparently, so was this Dalek.
We see the Doctor’s understandable prejudice towards this Dalek develop as the plot unfolds. He warns the museum’s collector against further antagonizing it, but also advises him to keep the Dalek imprisoned for fear of what it might do should it escape (knowing full well that the Dalek was in pain).
The origin of the Doctor’s prejudice, I believe, is two-fold. The first reason is obvious: he has seen what the Daleks can do and the destruction they can cause. He has experienced first-hand the hatred they feel for any living thing that is not Dalek. He has, therefore, no reason to expect that this particular Dalek will be anything less than a vicious killing machine. Indeed, on a basic level, the Dalek exactly that. The question then becomes whether, knowing the “evil” this being is capable of, it is right to cause it suffering. The Doctor believes so. Rose does not. Rose, in her apparent naivete, tries to help the Dalek and ends up in a rather dangerous situation. This brings us to the second reason for the Doctor’s prejudice: self-hatred. During their first encounter, the Dalek taunts the Doctor with their shared history: they are both Survivors. And they are both murderers, the Dalek of his enemies, the Doctor of his friends. Being so recently removed from the Time Wars, this reminder is particularly potent. The Doctor had to sacrifice his entire race to stop the universe from destroying itself, and he still feels an immense amount of guilt for it. This guilt and the resulting self-hatred and anger is compounded later by the Dalek’s forcing the Doctor into a similar choice: between Rose’s life and the lives of the people in the museum (and around the world, had it come to that). The Doctor hates the Dalek for putting him in that position, and hates himself for the decision he feels he has to make. Before, he had ordered them to keep the Dalek in captivity. Now that the Dalek is loose, he orders them to kill him if they can.
And yet, he has not taken the time to consider things from the Dalek’s perspective. As the last of his kind and a survivor of the Time Wars, the Dalek very probably was suffering from some post-traumatic stress similar to the Doctor’s. Add that to the torture and captivity, and you get one very unstable being. Normally, this would result in one of two things: the Dalek’s altered mental state would lead it to become incoherent, docile, and to abandon its destructive ways, or the Dalek would become even more of a crazed killer. Unfortunately for this particular Dalek, both things seem to happen at once and it goes through oscillations between thoughtless murder and scared posturing. And all the Doctor can see is the killing.
It has been said that Rose is the balance to the Doctor’s instability. She definitely serves that purpose here. Rose reminds the Doctor that all beings deserve a little common decency. She reveals to him through her actions that a paradigm is just a box in which to put ideas. Sometimes things, no matter how hard you try, do not fit nicely into that box. In the end, we discover that all the Dalek really wanted was to die with some dignity. He recognized the similarity between himself and the Doctor and realized that, between them, there were no sides of the War. They were just two unlucky survivors. Did that reconcile anything? No. But it did make the Doctor think about things in a way that wasn’t quite so black-and-white. It did let the Dalek die with dignity. And maybe, for all of that, some of the anger that comes with holding a grudge and living with guilt lessened a bit for the Doctor.