it builds on things which started in Genesis, when the Doctor pitches a 'thought problem' to Davros about wiping out all life in the universe, and Davros says 'I would do it' (and tries to do it in Journey's End, in a different way). Then we get the Dalek that kills itself rather than be human in the 2005 season; followed up by an elite group of Daleks who're allowed to think like Humans in Doomsday, one of whom tries to find a place for the Daleks as a normal species in Evolution (etc). And now, another of these enhanced Daleks makes the choice the Doctor couldn't make - to decide that the Daleks don't deserve to exist (flipping Dalek attitudes back on themselves).
And the Doctor still rejects that, but we get two Doctors so one can make the genocidal choice the Doctor rejected back in Genesis.