Daleks are the most consistent foe of the Doctor in the show's history and EU. They've forced many enemies of the Doctor to ally with him to fight these nasty pepperpots. But how come they can't always kill him? A few reasons, actually. read more
The Doctor will never kill off the Daleks, and the Daleks will never really kill the Doctor. As adversaries, they're just too good. It's pretty much the same reason the Borg won't wipe out the Federation and Stormtoopers can't hit the broad side of a barn: boring TV. read more
From what we saw in the new Doctor Who series(2005), Daleks cannot be taken out by humans with regular weaponry. The only instances we've seen Daleks die are: Rose Tyler disintegrating them. read more
You can't go back in time with the intent to change something. It's the classic time travel paradox. If he goes back in time to kill Davros, then the Daleks don't exist. But if the Daleks don't exist then the Doctor never goes back in time to kill Davros. You'd be trapped in a loop. read more