I'm not sure I'd say they're irredeemably evil; they're working from a completely different set of principles. Even the Doctor, who's about as anti-Dalek as he can be in this episode, states that "A Dalek is honest. It does what it was born to do for the survival of its species." The supremacy of the Daleks is the supreme good for a Dalek.

(While googling for the precise wording of that quote, I stumbled on this website (http://www.damaris.org/relessonsonline/block.php?b=409) which suggests using clips from the episode with students to explore the differences between the Dalek's/van Statten's/a Christian worldview.)